Check that multi-strings/CHOICE types don't use implicit tagging

Check that multi-strings/CHOICE types don't use implicit tagging

It never makes sense for multi-string or CHOICE types to use implicit
tagging since the content would be ambiguous. It is an error in the
template if this ever happens. If we detect it we should stop parsing.

Thanks to David Benjamin from Google for reporting this issue.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>