[Flang][OpenMP] Semantic checks for Atomic construct.

Patch implements restrictions from 2.17.7  of OpenMP 5.0 standard for atomic Construct. Tests for the same are added.

One of the restriction
`OpenMP constructs may not be encountered during execution of an atomic region.`
Is mentioned in 5.0 standard to be a semantic restriction, but given the stricter nature of parser in F18 it's caught at parsing itself.

This patch is a next patch in series from D88965.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D89583