[llvm] [mlir] [clang] [OpenMP] Migrate GPU Reductions CodeGen from Clang to OMPIRBuilder (PR #80343)

[Akash Banerjee via cfe-commits]

TIFitis wrote:

@jsjodin This PR only has the migration work. Flang/MLIR is left unchanged other than the few necessary changes.

Once this patch is merged, I'll put up a PR for having MLIR use these additions.

https://github.com/llvm/llvm-project/pull/80343