classes with destructor should error if one tries an explicit copy

[erickt]

class C {
    let x: int;
    new(x: int) { self.x = x; }

    drop {
        #error("dropping: %?", self.x);
    }
}

fn main() {
    let c = C(2);
    let d = copy c;
    #error("%?", d.x);
}

Even though the class has a destructor, it is valid rust code because it seems rust is ignoring the explicit copy because let d = copy c is the last use of c. I feel this is confusing, so I think rustc should report an error here.

[catamorphism]

This may become a non-issue when we stop using last-use to do optimization. I'm not completely sure.

[catamorphism]

Postponing until #2633 is resolved.

[catamorphism]

This is fixed now that #2633 is done. Test case in bbc46d5