[clang] [clang] Add `__bf16` Type Support Macros With Literal Suffix Support (PR #134214)

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overmighty wrote:

We could also add the following test cases:
- `bf16` suffix on integer literal.
- `bf16` suffix after another suffix.
- `bf16` suffix but no target support for bfloat16.

https://github.com/llvm/llvm-project/pull/134214