[clang] [HLSL] Get the index for resource bindings from the slot (PR #117303)

[Chris B via cfe-commits]

https://github.com/llvm-beanz approved this pull request.

This change looks right for DXIL (and I'll approve in that context), but @s-perron's comment about SPIR-V is something we should address before merging.

https://github.com/llvm/llvm-project/pull/117303