[clang] [llvm] [HLSL] Implement the `fwidth` intrinsic for DXIL and SPIR-V target (PR #161378)

[Alexander Johnston via llvm-commits]

Alexander-Johnston wrote:

I believe that's the nits fixed. I updated the `ddx_coarse-errors.ll` and ddy_coarse-errors.ll` tests to include a run with a compute triple and check for the appropriate error.

https://github.com/llvm/llvm-project/pull/161378