[AMDGPU] Add bf16 storage support

- [Clang] Declare AMDGPU target as supporting BF16 for storage-only purposes on amdgcn
  - Add Sema & CodeGen tests cases.
  - Also add cases that D138651 would have covered as this patch replaces it.
- [AMDGPU] Add BF16 storage-only support
  - Support legalization/dealing with bf16 operations in DAGIsel.
  - bf16 as a type remains illegal and is represented as i16 for storage purposes.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D139398