[flang][OpenACC] Lower parallel loop

Lower the `parallel loop` contrsuct and refactor some of the code
of parallel and loop lowering to be reused.

Also add tests for loop and parallel since they were not upstreamed.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D128510