[clang] Add a new flag -fexperimental-library to enable experimental library features

Based on the discussion at [1], this patch adds a Clang flag called
-fexperimental-library that controls whether experimental library
features are provided in libc++. In essence, it links against the
experimental static archive provided by libc++ and defines a feature
that can be picked up by libc++ to enable experimental features.

This ensures that users don't start depending on experimental
(and hence unstable) features unknowingly.

[1]: https://discourse.llvm.org/t/rfc-a-compiler-flag-to-enable-experimental-unstable-language-and-library-features

Differential Revision: https://reviews.llvm.org/D121141