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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for this! It's heading in the right direction. Some things which are still missing are;

* a release note in `clang/docs/ReleaseNotes.rst` so users know about the new suffix and macros.
* test coverage
  * `clang/test/Lexer/` should get a test for the literal suffix support
  * `clang/test/Preprocessor/` should get a test for the new predefined macros

As for the current test failures, they're both hitting the same failed assertion:
```
clang: /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-bt7k7-1/llvm-project/github-pull-requests/clang/lib/Frontend/InitPreprocessor.cpp:112: T PickFP(const llvm::fltSemantics *, T, T, T, T, T, T, T) [T = const char *]: Assertion `Sem == (const llvm::fltSemantics*)&llvm::APFloat::IEEEquad()' failed.
```
It almost seems as though `&TI.getBFloat16Format()` is returning something other than `&llvm::APFloat::BFloat()` for a target and thus it falls into the quad assertion while not being a quad semantics. You'll likely have to hook up a debugger to see what's going on there.

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