[HLSL] Add support for Root Signatures in Clang

[joaosaffran]

This issue parents the work related to handling root signatures in Clang.

Upon completed, it is expected we can specify a root signature attribute on an entry function, receive diagnostics from the semantic analysis of the attribute and then generate the appropriate LLVM IR metadata during front-end code generation.

As such, this work consists of parsing the root signature following the specified grammar, doing semantic analysis on the parsed attribute using the specified validations and to generate the LLVM IR metadata following the metadata schema. Each of these sub-tasks matches to one of the linked sub-issues and so this issue is resolved when all sub-tasks are completed.

For further information on the specification, please refer to this section of the HLSL Root Signature proposal.