[HLSL][DirectX] Handling semantic interpolation modifiers

This issue tracks the implementation work to add support for semantic interpolation modifiers.

It should implement the parsing and normalization of the modifiers as described in https://github.com/llvm/wg-hlsl/blob/main/proposals/0047-semantic-signatures-metadata-schema.md#interpolation-modifiers

AC:
- [ ] Add support for an `HLSLInterpolationModifierAttr` and ensure it parses into the corresponding metadata argument
- [ ] Add Sema Validation as outlined in the spec
- [ ] LIT testing of codegen for the parsing of attributes on the ast and also the metadata
- [ ] Unit testing is possible for the 'normalization' portion