P2944R3 Comparisons For `reference_wrapper`

[StephanTLavavej]

WG21-P2944R3 Comparisons For reference_wrapper
WG21-P3379R0 Constrain expected Equality Operators
LWG-4071 reference_wrapper comparisons are not SFINAE-friendly
LWG-4072 std::optional comparisons: constrain harder

Also tracked by #5873 (this is half "implement downlevel", half "patch C++26"):
LWG-4366 Heterogeneous comparison of expected may be ill-formed

Feature-test macros:

#define __cpp_lib_constrained_equality 202411L
#define __cpp_lib_reference_wrapper    202403L

Note: We are not yet accepting PRs for C++26 features.