enable two new clang-tidy checks

[Skylion007]

Description

I enabled a few new clang-tidy checks to harden the code bases. First, misc-definition-in-headers complains about headers issues that are likely to cause ODR problems when included from multiple translation units. The main one is forgetting to ensure that functions are all inline in headers. The 2nd check tries to use modernized for each loops. Luckily, we were already using these in pybind11 except for one test case which has been fixed in this PR. Finally, I changed a default setting for inefficient-string-concatenation to strict mode. We are already following strict mode, so this mainly will just prevent new issues from cropping up.

Suggested changelog entry:

* Enables clang-tidy checks misc-definitions-in-headers and modernize-loop-convert.