Swift: fix cartesian product in array readStep

Author: Robert Marsh

swift/ql/lib/codeql/swift/dataflow/internal/DataFlowPrivate.qll

@@ -705,7 +705,8 @@ predicate storeStep(Node node1, ContentSet c, Node node2) {
     node1.asExpr() = assign.getSource() and
     node2.(PostUpdateNode).getPreUpdateNode().asExpr() = subscript.getBase() and
     subscript = assign.getDest() and
-    subscript.getBase().getType().(InOutType).getObjectType() instanceof ArrayType
+    subscript.getBase().getType().(InOutType).getObjectType() instanceof ArrayType and
+    c.isSingleton(any(Content::ArrayContent ac))
   )
   or
   FlowSummaryImpl::Private::Steps::summaryStoreStep(node1.(FlowSummaryNode).getSummaryNode(), c,