[HLSL] Copy in/copy out parameter passing

[llvm-beanz]

Description

In HLSL parameter passing is copy-in/copy-out as specified by the in, inout and out parameter modifier keywords.

A proposal for how this will be implemented is documented in the Clang HLSL docs.

Deliverable

A full implementation of the solution in the document above and any required updates to the document.

Acceptance Criteria

• Documentation reflects the implementation
• Implementation is fully tested with codegen tests for DXIL and SPIR-V code generation.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Chris B (llvm-beanz)

## Description In HLSL parameter passing is copy-in/copy-out as specified by the `in`, `inout` and `out` parameter modifier keywords.

A proposal for how this will be implemented is documented in the Clang HLSL docs.

Deliverable

A full implementation of the solution in the document above and any required updates to the document.

Acceptance Criteria

• Documentation reflects the implementation
• Implementation is fully tested with codegen tests for DXIL and SPIR-V code generation.