[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,
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inbelic wrote:

I placed these arbitrarily here because there was some other msft related things locally.

Happy to move it somewhere more fitting

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