[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