[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.

Current conflict to resolve in planning:

Currently root signature parsing is in the Parse library. So we will need to know which RootSignature to parse during parse logic, as this can't currently be invoked in Sema. To move forward we need to either: move rootsignatureparser to happen during sema, or, be able to determine the entry point at parse time.

AC:

• Update the -T and -E options in clang-dxc
• Update the -T and -E options in clang and cc1
• Still in planning