C#: Cater for the dynamic member access call special case.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -1579,6 +1579,16 @@ private module ArgumentNodes {
           scope = def.getExpr()
         )
       else scope = e2
+      or
+      e1.(Argument).isArgumentOf(e2, _) and
+      e2 instanceof DynamicMemberAccess and
+      exists(AssignableDefinitions::AssignOperationDefinition def |
+        def.getTargetAccess() = e2 and
+        def.getExpr() = e1
+      ) and
+      exactScope = false and
+      isSuccessor = false and
+      scope = e1
     }
   }