[flang][OpenMP] Place the insertion point to the start of the block

After skeleton of the `Parallel Op` is created set the insertion point to start of the block. So that later `CodeGen` can proceed.

Note: This patch reflects the work that can be upstreamed from PR(merged)
PR: https://github.com/flang-compiler/f18-llvm-project/pull/424

Reviewed By: schweitz, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D88221