Generic parameters in some paths in patterns are ignored

The following compiles fine (the `Some::<u8>` line does not, however).

``` rust
fn main() {
    match Some("foo") {
        None::<int> => {}
        // Some::<u8>(_) => {}
        Some(_) => {}
    }
}  
```

(Nominated for backcompat-lang :( since changing this can cause programs to break.)
