[MLIR][OpenMP] Introduce the LoopWrapperInterface (#87232)

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.