[clang] [HLSL] Add Buffer def to frontend (PR #141086)

[Ashley Coleman via cfe-commits]

V-FEXrt wrote:

@inbelic From Justin, looks like it doesn't :) (and this is a bug) https://hlsl.godbolt.org/z/sqMMffh6K

I'm going to spin that off into its own task separate from this one so this doesn't get bogged down

https://github.com/llvm/llvm-project/pull/141086