Regression from stable: "Unexpected tail in unsized_info_ty"

There's been a regression in a recent nightly regarding dispatch on unsized types:

```rust
trait Foo {
    fn foo() { }
}

struct Bar;

trait Baz {
}

impl Foo for (Bar, Baz) { }


fn main() {
    <(Bar, Baz) as Foo>::foo()
}
```

https://is.gd/NjyQaO