C++: Handle using_container in using-enum downgrade script.

cpp/downgrades/25e365d1e8147df0f759b604f96eb4bffea48271/upgrade.properties

@@ -1,3 +1,4 @@
 description: Revert support for using-enum declarations.
 compatibility: partial
 usings.rel: run usings.qlo
+using_container.rel: run using_container.qlo

cpp/downgrades/25e365d1e8147df0f759b604f96eb4bffea48271/using_container.ql

@@ -0,0 +1,14 @@
+class UsingEntry extends @using {
+  string toString() { none() }
+}
+
+class Element extends @element {
+  string toString() { none() }
+}
+
+from UsingEntry u, Element parent, int kind
+where
+  usings(u, target, _, kind) and
+  using_container(parent, u) and
+  kind != 3
+select parent, u