
And constrain the new `operator==` since C++26. This patch implements parts of P2165R4, P2944R3, and a possibly improved resolution of LWG3882. Currently, libstdc++ and MSVC STL constrain the new overloads in the same way. Also set feature-test macro `__cpp_lib_constrained_equality` and add related release note, as P2944R3 will completed with this patch. Fixes #136765 Fixes #136770 Fixes #105424