[INF-0002] Add proposal for root signature handling in Clang

[damyanp]

This change adds an initial proposal for root signature handling in Clang.

This change proposes adding:

• New AST nodes to represent the root signature
• A metadata representation of the root signature so it can be stored in LLVM IR
• Validation and diagnostic generation for root signatures during semantic analysis
• Conversion of the metadata representation to the binary serialized format.

[damyanp]

Raw notes from a design review:

• [x] DXR State Objects - should be called HLSL Subobjects
• [x] The Decl is needed to support multiple types of root signature (local/global/attribute)
• [x] We do need to do local/global root signatures here to really understand how it the decl works in.
• [x] Idea: could we detect/optimize case if multiple entry points use the same root signature string? [optimization]
• [ ] Need details on error reporting and diagnostics - especially as this feeds into requirements for the data structures.
• [ ] Need details of how metadata binding to root signatures and entry points / subobjects work.
• [ ] Metadata that's a list of root signatures is useful? Maybe? Need to undertstand how it is used.
• [x] Example rootsig should have multiple descriptor ranges to be a good example.
• [x] Show how descriptor table's flags would be included.
• [x] All diagnostics that can be done in sema should be done there.
• [x] We can't re-use d3d code for this so we need to implement ourselves.

See below for issues filed for remaining issues.

[damyanp]

Remaining issues:

• [ ] #233
• [ ] #234