[clang] [llvm] Add cross builtins and cross HLSL function to DirectX and SPIR-V backend (PR #109180)

[Damyan Pepper via cfe-commits]

https://github.com/damyanp approved this pull request.

LGTM, and I see you've addressed all of the feedback.

Would be good to check @bogner is happy with the resolution of [this issue](https://github.com/llvm/llvm-project/pull/109180#discussion_r1781967896) before merging.

https://github.com/llvm/llvm-project/pull/109180