[clang] [llvm] [clang][HLSL] Add WaveIsFirstLane() intrinsic (PR #103299)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

We have this work tracked here: https://github.com/llvm/llvm-project/issues/99158

there should be some dxil specific tasks.

https://github.com/llvm/llvm-project/pull/103299