[HLSL] Root signature support

[llvm-beanz]

HLSL uses Root Signatures to specify the table of resources and other binding elements. In DirectX 12+ root signatures can be specified in HLSL source files using the [RootSignature(...)] attribute on entry functions and a signature description syntax defined in this documentation:

https://learn.microsoft.com/en-us/windows/win32/direct3d12/specifying-root-signatures-in-hlsl

Root Signatures are compiled into attributes embedded in the IR that then get encoded into runtime data structures which are read by the D3D runtime.

Support for Root Signatures specified via an attribute

• [ ] #55116
• [ ] AST node which save parsed root signature
• [ ] Parsing and diagnostic for the root signature
• [ ] Generate metadata in clang codeGen
• [ ] Serialization for emit RTS0 part
• [ ] Validate used resources exist in root signature at llvm IR level.
• [ ] Deserialization and print for writing test
• [ ] Testing
• [ ] Support for -T rootsig_1_0/rootsig_1_1 and -force-rootsig-ver rootsig_1_0
• [ ] Support for -frs

Root Signature Targets

• [ ] #99523

All diagnostics for Root Signatures

• [ ] Diagnostics that depend on bound resources

Local Root Signatures

• [ ] Support for local/global root signature

[davidcook-msft]

@llvm-beanz to break this into smaller work items to be refined later

[python3kgae]

Sub issues:

• [ ] AST node which save parsed root signature
• [ ] Parsing and diagnostic for the root signature
• [ ] Generate metadata in clang codeGen
• [ ] Serialization for emit RTS0 part
• [ ] Validate used resources exist in root signature at llvm IR level.
• [ ] Deserialization and print for writing test
• [ ] Testing
• [ ] Support for local/global root signature
• [ ] Support for -T rootsig_1_0/rootsig_1_1 and -force-rootsig-ver rootsig_1_0