[libcxx] cleans up __cpp_concepts mess

libc++ was previously a bit confused by what the value of __cpp_concepts
should be. Also replaces `__floating_point` with `floating_point` now
that it exists.

Differential Revision: https://reviews.llvm.org/D97015

GitOrigin-RevId: e4dd614ae81194b0a50361a91f8bd4364916ef2e
8 files changed