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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Hi! I'm wondering why this macro needs to be exposed outside of HLSL given that there's already `__SPIRV__` exposed for this exact purpose. These changes turn out to be disruptive in a downstream SYCL implementation (https://github.com/intel/llvm/issues/18219) because of a namespace named `__spirv`.

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