[clang] [llvm] [HLSL][SPIR-V] Add SV_DispatchThreadID semantic support (PR #82536)

[Justin Bogner via cfe-commits]

https://github.com/bogner approved this pull request.

LGTM. I don't like how we're choosing an intrinsic but changing that can come in as a follow up if you like.

https://github.com/llvm/llvm-project/pull/82536