[clang] [HLSL] cbuffer: Create host layout struct and add resource handle to AST (PR #122820)

[Chris B via cfe-commits]

https://github.com/llvm-beanz commented:

The one design question I have here is: should we be bothering to create a handle for the cbuffer in the frontend at all?

My thought had been that we would instead emit the cbuffer member declarations as constant address space (2 for DXIL) declarations and have a pass to collect them and replace them with accesses off a handle that we generate in the backend.

Then we just need to generate metadata in the frontend for the buffer's contained type, and which global variables are bound to which cbuffer.

https://github.com/llvm/llvm-project/pull/122820