[X86][RFC] Using `__bf16` for AVX512_BF16 intrinsics

This is an alternative of D120395 and D120411.

Previously we use `__bfloat16` as a typedef of `unsigned short`. The
name may give user an impression it is a brand new type to represent
BF16. So that they may use it in arithmetic operations and we don't have
a good way to block it.

To solve the problem, we introduced `__bf16` to X86 psABI and landed the
support in Clang by D130964. Now we can solve the problem by switching
intrinsics to the new type.

Reviewed By: LuoYuanke, RKSimon

Differential Revision: https://reviews.llvm.org/D132329