MFC r241077:

- Simplify the implementation of atomic_compare_exchange_strong_explicit.
- Evaluate the memory order argument in atomic_fetch_*_explicit macros.
- Implement atomic_store_explicit using atomic_exchange_explicit instead
  of a plain assignment.

MFC r241190:

Define clang feature test macro __has_extension. It's used in stdatomic.h.

MFC r254497:

Change the return type of the fallback implementation of the
atomic_compare_exchange_* macros in stdatomic.h to _Bool.