[Bug c/67432] Improve error message on empty enum

[chengniansun at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67432

--- Comment #4 from Chengnian Sun <chengniansun at gmail dot com> ---
(In reply to Marek Polacek from comment #3)
> Done.

This is very fast. Thanks, Marek.