rustc: impl on unknown type generates incorrect error

The code

``` rust
impl Foo {}
fn main() {}
```

generates the error

```
impl-bug.rs:1:6: 1:9 error: found module name used as a type: impl Foo::Foo (id=4)
impl-bug.rs:1 impl Foo {}
                   ^~~
```

but should instead give an error saying that `Foo` is not a known type. (This is the entire file, so no module named `Foo` is in scope.)
