Duplicate warnings for match_bool / single_match

The following snippet generates two lint warnings:

```
match test { //~ ERROR you seem to be trying to match on a boolean expression
    //~^ERROR you seem to be trying to use match
    false => { println!("Noooo!"); },
    _ => (),
};
```

FWIW, I think the right option is to skip the `single_match` lint if the `match_bool` lint already warned.
