[clang] [llvm] [HLSL][DirectX] Add the `Qdx-rootsignature-strip` driver option (PR #152196)

[Finn Plummer via llvm-commits]

inbelic wrote:

>From discussion, turning this back into a draft to investigate if we could have a `dx.codegenopts` or can avoid using metadata based on how some other options are used.

https://github.com/llvm/llvm-project/pull/152196