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

[Jan Leyonberg via cfe-commits]

jsjodin 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.

There is some code that is related to Flang/MLIR that shouldn't be in the patch. e.g. ReductionInfoManager is not used anywhere. I am currently working on a patch that refactors this code a bit and how it is used in the MLIR lowering.

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