[clang] [llvm] [HLSL][RootSignature] Define and integrate rootsig clang attr and decl (PR #137690)

[Finn Plummer via llvm-commits]

inbelic wrote:

Unused variables build failures were addressed [here ](https://github.com/llvm/llvm-project/pull/139590)and [here](https://github.com/llvm/llvm-project/commit/e74877bafebeff40f7213c199c8125bb49818fc1).

https://github.com/llvm/llvm-project/pull/137690