[HLSL] Root Signature support (non-library)

[damyanp]

This covers non-state object root signatures - the kind that can be associated with a non-library shader entry point - from specification via an HLSL attribute to inclusion in the final DX container's root signature blob.

General design documented at https://github.com/llvm/wg-hlsl/blob/main/proposals/0002-root-signature-in-clang.md