[Flang][OpenMP] Upstream the lowering of the parallel do combined construct

When parallel is used in a combined construct, then use a separate
function to create the parallel operation. It handles the parallel
specific clauses and leaves the rest for handling at the inner
operations.

Reviewed By: peixin, shraiysh

Differential Revision: https://reviews.llvm.org/D125465



Co-authored-by: Sourabh Singh Tomar <SourabhSingh.Tomar@amd.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Valentin Clement <clementval@gmail.com>
Co-authored-by: Nimish Mishra <neelam.nimish@gmail.com>