[clang] [HLSL] Implement SV_GroupThreadId semantic (PR #117781)

[Zhengxing li via cfe-commits]

lizhengxing wrote:

> Do we need to consider how spirv handles this? Otherwise, LGTM

This PR is similar to the one for SV_GroupID. We don't consider the spirv part for both PRs.

https://github.com/llvm/llvm-project/pull/117781