ICE with impl Foo for Foo for object safe Foo

``` rust
trait Foo {}

impl<'a> Foo for Foo+'a {}

fn main() {}
```

```
<anon>:3:1: 3:27 error: internal compiler error: coherence failed to report ambiguity: cannot locate the impl of the trait `Foo` for the type `Foo + 'a`
<anon>:3 impl<'a> Foo for Foo+'a {}
         ^~~~~~~~~~~~~~~~~~~~~~~~~~
```

cc #20940
