[clang] [llvm] [HLSL] [DXIL] Implement the `AddUint64` HLSL function and the `UAddc` DXIL op (PR #125319)

[Finn Plummer via cfe-commits]

https://github.com/inbelic commented:

My comments are addressed and this LGTM if we go ahead with the current implementation. That being said, I think if there is a (not-too-complex) way that we can re-use the codegen, then I would vote for that. So maybe we could bring this up quickly in a design meeting.

https://github.com/llvm/llvm-project/pull/125319