[libc++] [C++2b] [P1682] Add to_underlying.

* https://wg21.link/P1682

Reviewed By: ldionne, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D97365

GitOrigin-RevId: 43e421417378bab378eccdf88a66a0a46304fa13
9 files changed