C++: Support using enum declarations.

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -485,10 +485,17 @@ namespace_decls(
     int bodylocation: @location_default ref
 );
 
+case @using.kind of
+  1 = @using_declaration
+| 2 = @using_directive
+| 3 = @using_enum_declaration
+;
+
 usings(
     unique int id: @using,
     int element_id: @element ref,
-    int location: @location_default ref
+    int location: @location_default ref,
+    int kind: int ref
 );
 
 /** The element which contains the `using` declaration. */