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

[Nathan Gauër via cfe-commits]

Keenuts wrote:

Hi, I don't think there is an issue with only emitting the `__spirv__` macro with HLSL. Sent out https://github.com/llvm/llvm-project/pull/142401

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