Register declarations as well

Will be overriden by definitions.

src/cpp_enum.cpp

@@ -19,7 +19,7 @@ std::unique_ptr<cpp_enum_value> cpp_enum_value::build(const cpp_entity_index& id
 {
     auto result =
         std::unique_ptr<cpp_enum_value>(new cpp_enum_value(std::move(name), std::move(value)));
-    idx.register_entity(std::move(id), type_safe::ref(*result));
+    idx.register_definition(std::move(id), type_safe::ref(*result));
     return result;
 }