false positive on unused_parens lint

``` rust
fn main() {
    if (return) {}
}
```

sez:

```
<anon>:2:8: 2:16 warning: unnecessary parentheses around `if` condition, #[warn(unused_parens)] on by default
<anon>:2     if (return) {}
                ^~~~~~~~
```

However, removing the parentheses makes the file have illegal syntax, because `{}` is part of the return expression (so the `if` is missing its arm).
