Don't put the debug representation of a type into UB messages

[RalfJung]

This line is causing a bunch of trouble:

https://github.com/minirust/minirust/blob/f6bfad075def59ec52a1fed792a4c25f637d1e86/spec/lang/representation.md#L481

We probably want a small pretty-printer for types somewhere, because dumping the debug representation of an enum is entirely too much information and not readable.