[clang] [HLSL] Add __spirv__ macro (PR #132848)

[Steven Perron via cfe-commits]

https://github.com/s-perron commented:

Thanks. I was thinking of adding this myself, but it was pushed off. I don't know the best place to put it. I'll let others who know the FE review.

https://github.com/llvm/llvm-project/pull/132848