[HLSL][RootSignature] Add support for `rootsig-define` compiler flag

[inbelic]

This issue tracks implementing the DXC rootsig-define compiler option.

The behaviour should follow the specification here.

AC:

• Implement the rootsig-define driver option in clang-dxc
• Implement fdx-rootsignature-define driver option in clang and cc1
• Update ParseDeclCXX.cpp to invoke a parse of the root signature denoted with rootsig-define
• Update SemaHLSL.cpp to only attach an attribute if it is the entry function