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

[Nathan Gauër via cfe-commits]

Keenuts wrote:

Merging to unblock the structurizer work.
Let me know if you had a specific SEMA check in mind to add in the end!

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