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

[Johannes Doerfert via cfe-commits]

https://github.com/jdoerfert commented:

This patch is huge, and just skimming over it shows various places that could be split off, and others that should not make it to the review stage (commented out code).
Please update.

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