[MLIR][OpenMP] Update omp.wsloop translation to LLVM IR (4/5)

This patch introduces minimal changes to the MLIR to LLVM IR translation of
omp.wsloop to support the loop wrapper approach.

There is `omp.loop_nest` related translation code that should be extracted and
shared among all loop operations (e.g. `omp.simd`). This would possibly also
help in the addition of support for compound constructs later on. This first
approach is only intended to keep things running after the transition to loop
wrappers and not to add support for other use cases enabled by that transition.

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.