[HLSL] Generate AST for Root Signatures

[inbelic]

This issue handles "plugging-in" the root signature parser into the generation of the AST node. It consists of initializing an HLSLRootSignatureAttr with the parsed HLSLRootElements.

Root signatures are defined as a list of "root elements" and so the in-memory representation will retain this form.

This is dependent on #120472.

AC:

• Define HLSLRootSignatureAttr in Attr.td
• Use AdditionalMembers to hold an ArrayRef of HLSLRootElements
• Define handleHLSLRootSignature in SemaHLSL.h
• Implement handleHLSLRootSignature in SemaHLSL.cpp to simply invoke the parser and construct the Root Elements on the ASTContext, then set the AdditionalMembers to refence that
• Add test of a practical root signature example generated AST with ast-dump in AST/HLSL/RootSignatures-AST.hlsl

[inbelic]

Self-note for information regarding default values: https://learn.microsoft.com/en-us/windows/win32/direct3d12/specifying-root-signatures-in-hlsl

[damyanp]

Feedback from refinement: split this into two issues - one for parsing the root signature string, and then another for adding the attribute and hooking it into the parser.

[damyanp]

Needs some information about how ast-dump will be modified to allow for testing.