Improve "defining opaque type use does not fully define opaque type" error message

[varkor]

See https://github.com/rust-lang/rust/pull/66178/files#r344249868.

[nikomatsakis]

Example test:

#![feature(min_type_alias_impl_trait)]

type Foo = impl Copy;

enum Wrapper<T> {
    First(T),
    Second
}

fn produce() -> Wrapper<Foo> {
    Wrapper::First((22, Default::default()))
}

Outputs:

error[E0283]: type annotations needed
  --> src/lib.rs:11:25
   |
11 |     Wrapper::First((22, Default::default()))
   |                         ^^^^^^^^^^^^^^^^ cannot infer type
   |
   = note: cannot satisfy `_: Default`
   = note: required by `std::default::Default::default`

This seems ok. I'm not sure how we get to that code otherwise. I'm going to close the issue as "good enough".