[MLIR][OpenMP] Add omp.loop_nest operation

This patch introduces an operation intended to hold loop information associated
to the `omp.distribute`, `omp.simdloop`, `omp.taskloop` and `omp.wsloop`
operations. This is a stopgap solution to unblock work on transitioning these
operations to becoming wrappers, as discussed in
[this RFC](https://discourse.llvm.org/t/rfc-representing-combined-composite-constructs-in-the-openmp-dialect/76986).

Long-term, this operation will likely be replaced by `omp.canonical_loop`,
which is being designed to address missing support for loop transformations,
etc.