[clang] [AArch64][ARM] Treat __bf16 as vendor extension type for C++ name mangling substitution (PR #115956)

[John McCall via cfe-commits]

rjmccall wrote:

The ABI spec is pretty clear about this, and demangling is likely to get very screwed up because of the inconsistency.

I think should do the following:

- We should file a bug with GCC pointing out the problem.
- We should fix the bug under new `-fclang-abi-compat=` versions, but leave it around for old ones.
- We should add a release note.

https://github.com/llvm/llvm-project/pull/115956