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

[Akash Banerjee via cfe-commits]

TIFitis wrote:

@jsjodin @jdoerfert I have now added the missing support for complex variable reductions. Please let me know if this patch requires any other changes.

Thanks.

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