C++: Use mangled_name in ResolveClass.qll

The old code is still around to handle upgraded databases.

cpp/ql/src/semmlecode.cpp.dbscheme

@@ -705,6 +705,11 @@ usertype_uuid(
     unique string uuid: string ref
 );
 
+mangled_name(
+    unique int id: @declaration ref,
+    int mangled_name : @mangledname
+);
+
 is_pod_class(unique int id: @usertype ref);
 is_standard_layout_class(unique int id: @usertype ref);