`string_lit_as_bytes` gives incorrect recommendation with stringify!()

Sample program:

``` rust
#![feature(plugin)]
#![plugin(clippy)]

fn main() {
    let _ = bstringify!(foo).as_bytes();
}
```

Warning given:

> calling `as_bytes()` on a string literal. Consider using a byte string literal instead: `bstringify!(foo)`, #[warn(string_lit_as_bytes)] on by default

Of course, using `bstringify!(foo)` gives an undefined macro error from rustc.
