Provide adapter for std::reverse_iterator to replace boost::reverse_iterator for proxy reference iterators

[nvmkuruc]

Description of Change(s)

Depends on #2205, #2328

#2205 replaces std::reverse_iterator in several but not all instances. The C++ standard states std::reverse_iterator::operator-> should return the equivalent of std::addressof(operator*()). For iterators that return proxy types as references (say PcpNodeRef), taking the address of a temporary is a problematic operation. The C++20 standard modifies this specification to std::prev(/*underlying iterator*/).operator->(). Some compilers are already using the newer specification.

This change introduces Tf_ProxyReferenceReverseIterator as an adapter for std::reverse_iterator that specializes operator-> to use the C++20 specification, deferring all other operations to std::reverse_iterator. When possible, this is explicitly done with using directives. However, in many cases, the return and argument types require wrapper methods and functions.

This change depends on #2328 due to a bug with the current implementation of PcpNodeIterator and std::prev.

Fixes Issue(s)

• Prefer std::reverse_iterator instead of boost::reverse_iterator #2202

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[tallytalwar]

Filed as internal issue #USD-8122

[crevilla]

Since these three functions are the only ones that work as is from std::reverse_iterator, I'm wondering if it would be better to use a private inherit of std::reverse_iterator and just explicitly implement the pass throughs to the parent for these functions also. The extra protection against converting to or returning as the underlying STL implementation seems beneficial.