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

[Finn Plummer via cfe-commits]

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@@ -3619,6 +3619,19 @@ class Sema final : public SemaBase {
                                       SourceLocation NameLoc,
                                       bool IsTemplateTypeArg);
 
+  /// Computes the unique Root Signature identifier from the given signature,
+  /// then lookup if there is a previousy created Root Signature decl.
+  ///
+  /// Returns the identifier and if it was found
+  std::pair<IdentifierInfo *, bool>
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inbelic wrote:

If it is not found we still want to return it so that we don't need to re-compute it in `ActOnFinish...` when constructing the decl

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