Pull in r345199 from upstream libc++ trunk (by Petr Hosek):

Pull in r345199 from upstream libc++ trunk (by Petr Hosek):

  Revert "Teach __libcpp_is_floating_point that __fp16 and _Float16 are
  floating-point types."

  This reverts commits r333103 and r333108. _Float16 and __fp16 are C11
  extensions and compilers other than Clang don't define these for C++.

  Differential Revision: https://reviews.llvm.org/D53670

This prevents "_Float16 is not supported on this target" errors in
libc++'s type_traits header, in some cases.

Reported by:	Charlie Li
MFC after:	3 days