[HLSL] Implement user & system semantics

Proposal is now merged:
https://github.com/llvm/wg-hlsl/blob/main/proposals/0031-semantics.md

- [ ] Create HLSLUnparsedSemanticAttr & Attr hierarchy to support user semantics.
- [ ] Create vk::location attribute
- [ ] Migrate existing system semantics to use the added HLSLSemanticAttr class & move error handling.
- [ ] Implement location assignment for semantics & HLSLUserSemantics