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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I'm wary of deviating from GCC as that seems likely to introduce more ABI issues than it fixes. What do you think @rjmccall?

Also, this probably requires using ABI tags so users can opt in to the previous ABI behavior, assuming we want to move forward with changing the ABI.

And this definitely should come with a release note to let users know about the change in behavior.

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