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

[Steven Perron via cfe-commits]

https://github.com/s-perron approved this pull request.

This is useful because to will match the DXC behaviour, and make code more easier to port. Even if we have `__SPIRV__` is to good to have `__spirv__`.

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