[MLIR][OpenMP] Add cleanup region to omp.declare_reduction (#87377)

Currently, by-ref reductions will allocate the per-thread reduction
variable in the initialization region. Adding a cleanup region allows
that allocation to be undone. This will allow flang to support reduction
of arrays stored on the heap.

This conflation of allocation and initialization in the initialization
should be fixed in the future to better match the OpenMP standard, but
that is beyond the scope of this patch.