[mlir][OpenMP] Added assemblyFormat for atomic and critical operations

This patch adds assemblyFormat for `omp.critical.declare`, `omp.atomic.read`,
`omp.atomic.write`, `omp.atomic.update` and `omp.atomic.capture`.

Also removing those clauses from `parseClauses` that aren't needed
anymore, thanks to the new assemblyFormats.

Reviewed By: NimishMishra, rriddle

Differential Revision: https://reviews.llvm.org/D120248