[MLIR][OpenMP] Introduce the LoopWrapperInterface

This patch defines a common interface to be shared by all OpenMP loop wrapper
operations. The main restrictions these operations must meet in order to be
considered a wrapper are:

- They contain a single region.
- Their region contains a single block.
- Their block only contains another loop wrapper or `omp.loop_nest` and a
terminator.

The new interface is attached to the `omp.parallel`, `omp.wsloop`,
`omp.simdloop`, `omp.distribute` and `omp.taskloop` operations. It is not
currently enforced that these operations meet the wrapper restrictions, which
would break existing OpenMP loop-generating code. Rather, this will be
introduced progressively in subsequent patches.