[clang] [llvm] [HLSL][SPIRV][DXIL] Implement `dot4add_u8packed` intrinsic (PR #115068)

[Steven Perron via cfe-commits]

https://github.com/s-perron commented:

LGTM. I have not done enough in LLVM to know the etiquette yet. I wonder if the bug fix should be it own PR.

https://github.com/llvm/llvm-project/pull/115068