[libc++] Take advantage of -fexperimental-library in libc++

[libc++] Take advantage of -fexperimental-library in libc++

When -fexperimental-library is passed, libc++ will now pick up the
appropriate __has_feature flag defined by Clang to enable the
experimental library features.

As a fly-by, also update the documentation for the various TSes.

Differential Revision: https://reviews.llvm.org/D130176