[Clang] Add `-funstable` flag to enable unstable and experimental features

This new flag enables `__has_feature(cxx_unstable)` that would replace libc++ macros for individual unstable/experimental features, e.g. `_LIBCPP_HAS_NO_INCOMPLETE_RANGES` or `_LIBCPP_HAS_NO_INCOMPLETE_FORMAT`.

This would make it easier and more convenient to opt-in into all libc++ unstable features at once.

Differential Revision: https://reviews.llvm.org/D120160