[libc++] LWG3745 noexcept for atomic_wait.

The noexcept was already implemented, this only updates the synposis and
adds tests to validate that the functions are noexcept.

This implements:
- LWG3745 std::atomic_wait and its friends lack noexcept

Reviewed By: #libc, philnik

Differential Revision: https://reviews.llvm.org/D140575