When an associated type with GATs isn't specified in a `dyn Trait`, emit
an object safety error instead of only complaining about the missing
associated type, as it will lead the user down a path of three different
errors before letting them know that what they were trying to do is
impossible to begin with.
Fix#103155.
On the following example, point at `String` instead of the whole type:
```
error[E0277]: the trait bound `String: Copy` is not satisfied
--> $DIR/own-bound-span.rs:14:24
|
LL | let _: <S as D>::P<String>;
| ^^^^^^ the trait `Copy` is not implemented for `String`
|
note: required by a bound in `D::P`
--> $DIR/own-bound-span.rs:4:15
|
LL | type P<T: Copy>;
| ^^^^ required by this bound in `D::P`
```
