dropck is unsound with closure upvars

[arielb1]

This should be fixed by @nikomatsakis's new patch even if the code isn't changed, but I thought it would be nice to track too.

use std::fmt;
struct NoisyDrop<T: fmt::Debug>(T);
impl<T: fmt::Debug> Drop for NoisyDrop<T> {
    fn drop(&mut self) {
        let _ = vec!["0wned"];
        println!("dropping {:?}", self.0)
    }
}
fn foo() {
    let (w,v);
    v = vec![0usize];
    w = {
        let u = NoisyDrop(&v);
        move || u.0.len()
    };
}

fn main() { foo() }

After this, the only non-trivial variants dropck correctly handles are TySlice and TyTuple - I guess even a stopped clock is right twice a day.