P2781R9 `constant_wrapper`

[StephanTLavavej]

WG21-P2781R9 constant_wrapper
LWG-4351 integral-constant-like needs more remove_cvref_t
LWG-4383 constant_wrapper's pseudo-mutators are underconstrained

Feature-test macro (expected):

#define __cpp_lib_constant_wrapper 202506L

Note: We're focused on implementing the remaining library-only features in C++23. Until that's done, we will NOT be accepting PRs for C++26 features.