[OpenMP][MLIR]Add support for guided, auto and runtime scheduling

When using parallel loop construct, the OpenMP specification allows for
guided, auto and runtime as scheduling variants (as well as static and
dynamic which are already supported).

This adds the translation from MLIR to LLVM-IR for these scheduling
variants.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D101435