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

[Finn Plummer via cfe-commits]

inbelic wrote:

> I didn't see `SemaHLSL.cpp` changes, I saw the tests though.

Since the function signature is completely captured in `hlsl_intrinsics.h` we aren't required to manually define it in `SemaHLSL.cpp`

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