Auto merge of #29962 - aturon:coherence-errors, r=nmatsakis

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

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