[flang][OpenMP] Support reduction of allocatable variables (#88392)

Both arrays and trivial scalars are supported. Both cases must use
by-ref reductions because both are boxed.

My understanding of the standards are that OpenMP says that this should
follow the rules of the intrinsic reduction operators in fortran, and
fortran says that unallocated allocatable variables can only be
referenced to allocate them or test if they are already allocated.
Therefore we do not need a null pointer check in the combiner region.