Shouldn't be able to let-pattern-unwrap structs with dtors

[bblum]

struct X {
    x: ~str;
    drop {
        error!("destructing wrapper");
        error!("value: %s", self.x);
    }
}

fn foo(+x: X) -> ~str {
    let X { x: y } = x;
    error!("unwrapped it");
    y
}

fn main() {
    let x = X { x: ~"hello" };
    let y = foo(x);
    error!("got inner");
    error!("contents: %s", y);
}

prints:

rust: ~"unwrapped it"
rust: ~"destructing wrapper"
Segmentation fault (core dumped)

[bblum]

oops, dup of #3147