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

[Farzon Lotfi via llvm-commits]

farzonl wrote:

forgot to mention this on the ddx_coarse\ddy_coarse pr but in DXIL.td you defined those two opcodes as pixel and library shader but we didn't write a test to confirm they would error for say a use on a compute shader. Can you add those two tests to this pr?

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