P3372R3 constexpr Containers And Adaptors

[StephanTLavavej]

WG21-P3372R3 constexpr Containers And Adaptors

We'll need to find a solution for:

• <xhash>: Uses of _Reinterpret_move_iter and _Assign_cast don't seem right #5314

Feature-test macros (expected):

#define __cpp_lib_constexpr_deque 202502L
#define __cpp_lib_constexpr_flat_map 202502L
#define __cpp_lib_constexpr_flat_set 202502L
#define __cpp_lib_constexpr_forward_list 202502L
#define __cpp_lib_constexpr_list 202502L
#define __cpp_lib_constexpr_map 202502L
#define __cpp_lib_constexpr_queue 202502L
#define __cpp_lib_constexpr_set 202502L
#define __cpp_lib_constexpr_stack 202502L
#define __cpp_lib_constexpr_unordered_map 202502L
#define __cpp_lib_constexpr_unordered_set 202502L

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.