[libc++] [C++2b] [P1048] Add is_scoped_enum and is_scoped_enum_v.

* https://wg21.link/p1048

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D94409

GitOrigin-RevId: 1f1250151f222ba391d05dcc173f4b6c65d05ca2
8 files changed