Traits with associated types appearing on structs, enums or other traits

[nikomatsakis]

Currently the pseudo-parameters for associated types are only created on impls and fns. This means that examples like #17732 don't work.

This issue is strictly limited to the acyclic case. The cyclic case is covered by #18436

[jroesch]

cc me

[nikomatsakis]

Not really relevant to the new implementation. Closing.