[clang] [llvm] Add length HLSL function to DirectX Backend (PR #101256)

[Farzon Lotfi via cfe-commits]

https://github.com/farzonl requested changes to this pull request.

you need a `llvm/test/CodeGen/DirectX/length_error.ll file.  sqrt doesn't support double.

https://github.com/llvm/llvm-project/pull/101256