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

[Jan Leyonberg via cfe-commits]

jsjodin wrote:

I think the patch should be split up into at least 2 pieces. First would be the migration work and changes to Clang, the second would be working on adding reduction support for flang. 

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