[mlir][OpenMP] Added assemblyFormat for ParallelOp

This patch adds assemblyFormat for omp.parallel operation.

Some existing functions have been altered to fit the custom directive
in assemblyFormat. This has led to their callsites to get modified too,
but those will be removed in later patches, when other operations get
their assemblyFormat. All operations were not changed in one patch for
ease of review.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D120157