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

[Alexander Johnston via llvm-commits]

Alexander-Johnston wrote:

I don't have permissions to merge this. If you're happy with the nit fixes and test changes can you merge it @farzonl ? Thanks!

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