[HLSL][RootSignature] Add support for root signature as a target

[inbelic]

This issue tracks implementing DXC's ability to use a root signature versions as a target with a root signature as an entry point.

The behaviour should follow the specification here.

AC:

• Update the -T and -E options in clang-dxc
• Update the -T and -E options in clang and cc1
• Create a FrontendAction wrapper around ASTFrontendAction that will parse + inject the root signature (and maybe empty function if required) into the AST and then short-circuit any further parsing

[inbelic]

Determining the path forward is blocked on how we implement #150274.

[inbelic]

Marking as unblocked now that I have a plan forward with mentioned issue. Both issues will both introduce a frontend action but are independent.