More descriptive error when qself path doesnt have a trait on the RHS of `as`
`<Ty as Enum>::Assoc` should report that `Enum` is a trait. Main question is whether to eagerly report the error, or raise it with `return Err(..)` -- i'll note that in an inline comment though.
cc `@GuillaumeGomez` who said this came up at a Paris Rust meetup.
r? `@petrochenkov`
23ee2af2f7