An enum type is kind of like a struct or union type, in that field
errors are happening during type resolution. The only difference is that
type resolution happens at the time the type is created. So, errors in
fields should not cause the type to be deleted: we've already added a
reference entry, and incremenetal dependencies which must be invalidated
if the compile error is fixed. Once we call `WipEnumType.prepare`, we
should never call `WipEnumType.cancel`. This is analagous to logic for
enum declarations in `Sema.zirEnumDecl`.