Patch implements restrictions from 2.17.7 of OpenMP 5.0 standard for atomic Construct. Tests for the same are added.
There is a todo for Hint clause restriction which I was unable to figure out as Hint is currently not found in `actualClauses` list.
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.