[Clang][FP16] Add 4 builtins for _Float16

We are lacking builtins support for `_Float16`. In most cases, we can use other floating-type builtins and truncate them to `_Float16`.
But it's a problem to SNaN, e.g., https://gcc.godbolt.org/z/cqr5nG1jh
This patch adds `__builtin_nansf16` support as well as other 3 ones since they are usually used together.

Reviewed By: LuoYuanke

Differential Revision: https://reviews.llvm.org/D127050