Destructors do not run when implemented on unit-like structs

[bstrie]

struct Bomb;

impl Drop for Bomb {
    fn finalize(&self) {
        println("Somebody set up us the bomb!");  // this is never printed
    }
}

fn main() {
    let b = Bomb;
    println(fmt!("b = %?", b));
}

Adding a single field like { a: int } to the struct makes it print the expected message.

[luqmana]

Fixed by #6889.