[clang] [llvm] [mlir] [openmp] [OpenMP][offload] Cross-team reductions with variable number of teams (PR #195102)

[Robert Imschweiler via cfe-commits]

ro-i wrote:

(I rebased after adapting the tests, so there are a few that are not adapted yet, I'll fix them after dinner)

https://github.com/llvm/llvm-project/pull/195102