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

[Chris B via cfe-commits]

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@@ -5178,6 +5179,29 @@ class HLSLBufferDecl final : public NamedDecl, public DeclContext {
   friend class ASTDeclWriter;
 };
 
+class HLSLRootSignatureDecl final : public NamedDecl {
+  ArrayRef<llvm::hlsl::rootsig::RootElement> RootElements;
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llvm-beanz wrote:

This should probably be a `TrailingObjects` instead of an `ArrayRef` so that it is allocated with the Decl (and in adjacent cache-friendly memory).

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