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

[via cfe-commits]

shraiysh wrote:

Not super familiar with the work here, but could we maybe split it as - one patch for OpenMPIRBuilder changes with tests in OpenMPIRBuilderTest.cpp and then second patch for the clang changes to use those OpenMPIRBuilder changes? Would the CI tests cause issues?

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