[Clang][BFloat16] Upgrade __bf16 to arithmetic type, change mangling, and...

[Clang][BFloat16] Upgrade __bf16 to arithmetic type, change mangling, and extend excess precision support

Pursuant to discussions at
https://discourse.llvm.org/t/rfc-c-23-p1467r9-extended-floating-point-types-and-standard-names/70033/22,
this commit enhances the handling of the __bf16 type in Clang.
- Firstly, it upgrades __bf16 from a storage-only type to an arithmetic
  type.
- Secondly, it changes the mangling of __bf16 to DF16b on all
  architectures except ARM. This change has been made in
  accordance with the finalization of the mangling for the
  std::bfloat16_t type, as discussed at
  https://github.com/itanium-cxx-abi/cxx-abi/pull/147.
- Finally, this commit extends the existing excess precision support to
  the __bf16 type. This applies to hardware architectures that do not
  natively support bfloat16 arithmetic.
Appropriate tests have been added to verify the effects of these
changes and ensure no regressions in other areas of the compiler.

Reviewed By: rjmccall, pengfei, zahiraam

Differential Revision: https://reviews.llvm.org/D150913