regression: ICE on an unconfigured interpolated item in a macro invocation

For example, the following causes an ICE:

``` rust
macro_rules! m {
    ($i:item) => {
        error! {
            $i //< This becomes an unconfigured interpolated item in the above macro invocation
        }
    }
}

fn main() {
    m!(#[cfg(any())] fn g() {}); //< in this expansion.
}
```
