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

[Justin Bogner via cfe-commits]

https://github.com/bogner commented:

A couple of minor comments. The direction seems reasonable to me, but it'd be good to get someone more familiar with the Parser and AST to weigh in.

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