Currently, a coherence error based on overlapping impls simply mentions
the trait, and points to the two conflicting impls:
```
error: conflicting implementations for trait `Foo`
```
With this commit, the error will include all input types to the
trait (including the `Self` type) after unification between the
overlapping impls. In other words, the error message will provide
feedback with full type details, like:
```
error: conflicting implementations of trait `Foo<u32>` for type `u8`:
```
When the `Self` type for the two impls unify to an inference variable,
it is elided in the output, since "for type `_`" is just noise in that
case.
Closes#23980
r? @nikomatsakis
38da1a4064