[MLIR][SCF] Update scf.parallel lowering to OpenMP (3/5)

This patch makes changes to the `scf.parallel` to `omp.parallel` + `omp.wsloop`
lowering pass in order to introduce a nested `omp.loop_nest` as well, and to
follow the new loop wrapper role for `omp.wsloop`.

This PR on its own will not pass premerge tests. All patches in the stack are
needed before it can be compiled and passes tests.