Avoid stack overflow when checking enum representability

It was possible to trigger a stack overflow in rustc because the routine used to verify enum representability,
type_structurally_contains, would recurse on inner types until hitting the original type. The overflow condition was when a different structurally recursive type (enum or struct) was contained in the type being checked.

Fixes issue #3008.

I suspect my solution isn't as efficient as it could be. I pondered adding a cache of previously-seen types to avoid duplicating work (if enums A and B both contain type C, my code goes through C twice), but I didn't want to do anything that may not be necessary.

I'm a new contributor, so please pay particular attention to any unidiomatic code, misuse of terminology, bad naming of tests, or similar horribleness :)