[flang][OpenMP] Added initial support for lowering OpenMP parallel construct

This patch lower `!OMP PARALLEL` construct from PFT to OpenMPDialect operations.
This is first patch in this direction(lowering parallel construct).

OpenMP parallel construct can have multiple clauses and parameters. This patch
only implements lowering of an empty(contains no code in body) parallel construct
without any clauses or parameters.

Patch is carved out of following approved PR:
https://github.com/flang-compiler/f18-llvm-project/pull/322

Reviewed By: kiranchandramohan, DavidTruby

Differential Revision: https://reviews.llvm.org/D84965