[libc++] Make feature-test macros consistent with availability macros

Before this patch, feature-test macros didn't take special availability
markup into account, which means that feature-test macros can sometimes
appear to "lie". For example, if you compile in C++20 mode and target
macOS 10.13, the __cpp_lib_filesystem feature-test macro will be provided
even though the <filesystem> declarations are marked as unavailable.
This patch fixes that.

rdar://68142369

Differential Revision: https://reviews.llvm.org/D94983