[clang] [llvm] [HLSL][RootSignature] Implement parsing of `RootParamter`s (PR #121803)

[Finn Plummer via llvm-commits]

inbelic wrote:

Ignore the first commit, it is simply included to build until https://github.com/llvm/llvm-project/pull/121799 is merged.

https://github.com/llvm/llvm-project/pull/121803