`impl Undefined {}` gives `impl` coherence error, not unresolved name

``` rust
impl Undefined {}

fn main() {}
```

```
<anon>:1:6: 1:15 error: inherent implementations are not allowed for types not defined in the current module.
<anon>:1 impl Undefined {}
              ^~~~~~~~~
```

This should say `error: use of undeclared type name `Undefined`` like `fn foo() -> Undefined {}` does.
