[mlir] Add translation of omp.wsloop to LLVM IR

Introduce a translation of OpenMP workshare loop construct to LLVM IR. This is
a minimalist version to enable the pipeline and currently only supports static
loop schedule (default in the specification) on non-collapsed loops. Other
features will be added on per-need basis.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D92055