[MLIR][OpenMP] Add omp.loop_nest operation (#87083)

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.