[libc++] Consistency on _LIBCPP_CLANG_VER tests in <type_traits>.

[libc++] Consistency on _LIBCPP_CLANG_VER tests in <type_traits>.

This came out of my review comments on D97283.

This patch re-enables the use of `__is_fundamental`, `__is_signed`, etc.
on non-Clang compilers. Previously, when we found that a builtin didn't
work on old Clangs, we had been reacting by limiting its use to new Clangs
(i.e., we'd also stop using it on new GCCs and new MSVCs, just because of
the old Clang bug). I claim that this was unintentional.

Notice that on Apple Clang, `_LIBCPP_COMPILER_CLANG` is defined and
`_LIBCPP_CLANG_VER` is not defined (therefore `0` in arithmetic expressions).
We assume that Apple Clang has all the bugs of all the Clangs.

Differential Revision: https://reviews.llvm.org/D98720