Lang item invalid code compiles

The following code compiles whereas it shouldn't:

``` Rust
#![feature(lang_items)]

#[lang = "cookie"]
fn cookie() -> ! {
    loop {}
}

#[lang = "cookie"]
fn cookie2() -> ! {
//~^ ERROR: Duplicate lang item found: `cookie`.
//~| NOTE First definition of `cookie` here.
    loop {}
}

fn main() {}
```

cc @brson 
#31957
