[libc++] Prefer __has_builtin for detecting compiler-provided type_traits

Both clang and GCC support using `__has_builtin` for detecting compiler-provided type_traits. Use it instead of `__has_keyword` or `__has_feature` to remove special-casing for GCC-provided builtins

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D129056