fix(clang-tidy): Apply all performance fixes to tests and enable performance checks on CI

[Skylion007]

Description

@rwgk @henryiii I went ahead and tried to fix or ignore as many of the performance issues in tests as possible. It's a big PR, but by fixing all the issues in tests or ignoring them explicitly, this should allow us to automatically run the performance checks on CI. Feel free to directly edit this PR if I broke any tests.
I also caught a couple of issues in the core library that weren't caught by the automated fixes earlier.

Types of changes
I was very conservative with what changes I made manually since I didn't want to break any testing behavior.
The changes made are mostly as follows:

1. Add or removing missing std::move
2. Adding noexcept
3. Change certain variables to const ref. It's a very noisy error in tests, but it was also by far the most common performance error throughout the codebase, so I left this check enabled. I tried to fix it in several places, but others I left it as NOLINTNEXTLINE to try to avoid changing behavior of the test. Occasionally, changing it to a const ref broke things anyway due to typing errors. Many of these NOLINTNEXTLINES should probably be migrated over to proper const ref fixes if safe, but I'll let @rwgk, @henryiii or someone else more familiar with the tests down the line fix them later. It's not very high pri since testing isn't that performance sensitive. I tried to change it in examples that people might copy from test to try to demonstrate best coding practices. In other words, NOLINTNEXTLINE is the default for any test that caused issues when changing it to a const ref or I was uncertain whether it would change testing behavior. I also added it in args that are likely to be elided like static methods in tests to avoid having to change all those by hand. Feel free to get rid of as many of these as possible @rwgk .
   Some of the unparameterized nolintnextlines was because changing the class to be a multiline class or adding std::move in the constructor initializer list would break the pygrep pre-commit hook for proper upper/lowercasing.

@rwgk Plenty of tricky decisions, the most common one is not fixing the Cat and Dog classes in tests because it would cause a formatting change that would break the pygrep pre-commit hook. The other tricky decisions was mostly whether to actually fix the pass by value or leave it as a NOLINT. I erred on the side of caution with NOLINT when possible to avoid breaking tests.

Suggested changelog entry:

* Enable clang-tidy performance checks throughout the codebase to enforce performant coding practices.

[rwgk]

That's fantastic! I glanced through the changes but didn't go through with a comb, thinking that's not needed, given that this PR allows us to add automatic tooling for code health, and that we already have extensive testing. As far as I saw there are mostly or only (?) three types of changes:

1. foo -> const foo&
2. adding noexcept
3. NOLINTNEXTLINE

Did I miss something important? What do you think about adding that list + anything I missed to the PR description? If you could explain (outline is fine) the rationale for deciding between adding const & vs NOLINTNEXTLINE that would be a plus. If there are any special or tricky decisions you made, pointing those out would be good, too. I will take a closer look at those.

[Skylion007]

@rwgk Feel free to edit the PR directly. I pointed out some of the tricky decisions I made. Mostly erred on the side of caution and just NOLINTed things instead of changing behavior.

[Skylion007]

@rwgk I also wanted to enable readability-const-return-type , but it generated a bunch of errors I am not sure how to fix such as:

../include/pybind11/detail/descr.h:77:24: error: return type 'decltype(int_to_str<1UL / 10, 1UL % 10>::digits)' (aka 'const pybind11::detail::descr<1>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
../include/pybind11/detail/descr.h:77:24: error: return type 'decltype(int_to_str<2UL / 10, 2UL % 10>::digits)' (aka 'const pybind11::detail::descr<1>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
../include/pybind11/detail/descr.h:77:24: error: return type 'decltype(int_to_str<32UL / 10, 32UL % 10>::digits)' (aka 'const pybind11::detail::descr<2>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
../include/pybind11/detail/descr.h:77:24: error: return type 'decltype(int_to_str<3UL / 10, 3UL % 10>::digits)' (aka 'const pybind11::detail::descr<1>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
../include/pybind11/detail/descr.h:77:24: error: return type 'decltype(int_to_str<4UL / 10, 4UL % 10>::digits)' (aka 'const pybind11::detail::descr<1>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
../include/pybind11/detail/descr.h:77:24: error: return type 'decltype(int_to_str<5UL / 10, 5UL % 10>::digits)' (aka 'const pybind11::detail::descr<1>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
../include/pybind11/detail/descr.h:77:24: error: return type 'decltype(int_to_str<64UL / 10, 64UL % 10>::digits)' (aka 'const pybind11::detail::descr<2>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
../include/pybind11/detail/descr.h:77:24: error: return type 'decltype(int_to_str<6UL / 10, 6UL % 10>::digits)' (aka 'const pybind11::detail::descr<1>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
/Users/agokaslan/git/my_forks/pybind11/tests/test_eigen.cpp:180:9: error: return type 'const Eigen::MatrixXd' (aka 'const Matrix<double, Dynamic, Dynamic>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
        static const Eigen::MatrixXd createConst() { return Eigen::MatrixXd::Ones(10, 10); }
        
../include/pybind11/detail/../pytypes.h:642:5: error: return type 'pybind11::detail::generic_iterator<pybind11::detail::iterator_policies::dict_readonly>::reference' (aka 'const pair<pybind11::handle, pybind11::handle>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
    reference operator*() const { return Policy::dereference(); }
    ^
../include/pybind11/detail/../pytypes.h:642:5: error: return type 'pybind11::detail::generic_iterator<pybind11::detail::iterator_policies::sequence_fast_readonly>::reference' (aka 'const pybind11::handle') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
../include/pybind11/detail/../pytypes.h:686:5: error: return type 'pybind11::detail::iterator_policies::sequence_fast_readonly::reference' (aka 'const pybind11::handle') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
    reference dereference() const { return *ptr; }
    ^
../include/pybind11/detail/../pytypes.h:730:5: error: return type 'pybind11::detail::iterator_policies::dict_readonly::reference' (aka 'const pair<pybind11::handle, pybind11::handle>') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
    reference dereference() const { return {key, value}; }
    ^
../include/pybind11/detail/../pytypes.h:882:5: error: return type 'pybind11::iterator::reference' (aka 'const pybind11::handle') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]
    reference operator*() const {

[rwgk]

Hi Aaron, thanks for all the comments! I don't have the free bandwidth to get deeply into the weeds. My suggestion is to turn those github comments into terse code comments to go with the NOLINTNEXTLINE. Then over time we can weed them out, e.g. when someone is working on a related change and has built up the required context in their head already. And the cases that hang around probably aren't that important! What seems much more important to me is to get the automatic runs up and running.

[rwgk]

@rwgk I also wanted to enable readability-const-return-type , but it generated a bunch of errors I am not sure how to fix

No immediate idea. Sorry I don't know much about this code, too. My suggestion is to get this PR merged, then open a separate one for this issue while we have the rest in place already.