[libc++] P3372R3: constexpr `unordered_map`

[frederick-vs-ja]

Make unordered_map constexpr as part of P3372R3.

Note that node-handle::key() is intentionally left not constexpr, and the constexpr-ness largely requires a user-defined hash functor.

Feature-test macro (also covers constexpr unordered_multimap):

#define __cpp_lib_constexpr_unordered_map 202502L // also in <unordered_map>