[clang] [NFC][HLSL] Move Sema work from `ParseMicrosoftRootSignatureAttributeArgs` (PR #143184)

[Aaron Ballman via cfe-commits]

================
@@ -653,6 +654,29 @@ ParsedType Sema::ActOnMSVCUnknownTypeName(const IdentifierInfo &II,
   return CreateParsedType(T, Builder.getTypeSourceInfo(Context, T));
 }
 
+std::pair<IdentifierInfo *, bool>
+Sema::ActOnStartRootSignatureDecl(StringRef Signature) {
+  auto Hash = llvm::hash_value(Signature);
+  std::string IdStr = "__hlsl_rootsig_decl_" + std::to_string(Hash);
+  IdentifierInfo *DeclIdent = &(getASTContext().Idents.get(IdStr));
+
+  // Check if we have already found a decl of the same name
----------------
AaronBallman wrote:

```suggestion
  // Check if we have already found a decl of the same name.
```

https://github.com/llvm/llvm-project/pull/143184