Compiler silently ignores #[warn(..)] attributes where non-applicable

The same ICFP (#2928) teammate as in bug #2942 was also confused by this code, which warns about the implicit copy on x:

```
enum asdf = int;
impl foo for asdf {
    #[warn(no_implicit_copies)]
    fn foo(x: ~[asdf]) -> ~[asdf] { x } 
}
fn main() {
    asdf(5).foo(~[]);
}
```

Moving the #[warn(..)] up to the top level makes it compile with no warnings.
