"type parameter `A` is unused" span in `type Foo<A> = u8;` should point to A

It currently points to the RHS of the `type`.

``` rust
type Foo<A> = u8;

fn main() {}
```

```
<anon>:1:15: 1:17 error: type parameter `A` is unused [E0091]
<anon>:1 type Foo<A> = u8;
                       ^~
```
