Ensure enums generate a ref in owning structs

Ensure enums generate a ref in owning structs

Previously we only added a ref in an owning struct to an enum if
that enum contained a trait in a field. This isn't sufficient as an
enum could still contain a reference to an opaque object, which
needs to stick around as long as the enum exists.

For simplicity, simply add a ref in all cases as it seems to still
work in our own internal tests.