[libc++] P3372R3: constexpr `unordered_set`

[frederick-vs-ja]

Make unordered_set constexpr as part of P3372R3.

Note that the constexpr-ness largely requires a user-defined hash functor.

Feature-test macro (also covers constexpr unordered_multiset):

#define __cpp_lib_constexpr_unordered_set 202502L // also in <unordered_set>