[HLSL] Add frontend test coverage of Root Signatures to Offload Test Suite

[inbelic]

This issue is used to define a baseline expected behaviour of how root signatures are handled with DXC, which will be used for reference when defining the expected behaviour and implementation of root signatures into Clang defined here.

It is an investigative issue to find and provide various root signatures for testing.

AC:

• Coverage of specifying Root Signatures, as an entry-function attribute, is added to the offload test suite