[clang] Reland "[HLSL][RootSignature] Define and integrate HLSLRootSignatureAttr" (PR #134293)

[Finn Plummer via cfe-commits]

inbelic wrote:

Closing as this would actually introduce a circular dependency between clangParse and clangSema, as surprisingly, clangParse has a dependency on clangSema. This will need to reland after we move the HLSLRootSignatureParser into an accessible location (most likely within clangSema itself)

https://github.com/llvm/llvm-project/pull/134293