Incorrect "type alias is never used"

```
struct Foo;

type Bar = Foo;

impl Bar {
    fn baz(&self) {}
}

fn main() {
    Foo.baz();
}
```

```
rustc 1.17.0 (56124baa9 2017-04-24)
warning: type alias is never used: `Bar`
 --> <anon>:3:1
  |
3 | type Bar = Foo;
  | ^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default
```