It doesn't make sense to warn about a switch case not being an
enumerator if the enum type has no enumerators; such a type is being
used as an opaque typedef rather than a true enumerated type.  This
usage makes more sense with C++11 enums with explicitly specified
underlying type, but I don't think it's necessary to limit the patch
to that case.

Tested x86_64-pc-linux-gnu, applying to trunk.