[MLIR][SCF] Update scf.parallel lowering to OpenMP (3/5) (#89212)

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.