`Ty::ty_to_def_id` does not yield all possible `DefId`s that it could

[oli-obk]

I don't know if this is really problem, but e.g. the ReachEverythingInTheInterfaceVisitor reimplements this function (and yields a bunch of other DefIds, too.

This needs some looking at

Mostly leaving this here so I don't forget

[eddyb]

ty_to_def_id is probably a hack for cache keying, and should ideally be removed?