Provide overlapping types for coherence errors

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

src/test/compile-fail/coherence-default-trait-impl.rs

@@ -15,7 +15,7 @@ trait MyTrait {}
 impl MyTrait for .. {}
 
 impl MyTrait for .. {}
-//~^ ERROR conflicting implementations for trait `MyTrait`
+//~^ ERROR conflicting implementations of trait `MyTrait`
 
 trait MySafeTrait {}