librustc: Add a hack to allow unboxed closures to achieve the 'static

[pcwalton]

bound if possible.

Closes #16560.

r? @nikomatsakis

[lilyball]

This patch fixes a bit of test code that doesn't compile on latest master:

#![feature(unboxed_closures)]

struct Dropper {
    msg: &'static str
}

impl Dropper {
    fn new(msg: &'static str) -> Dropper {
        Dropper {
            msg: msg
        }
    }
}

impl Drop for Dropper {
    fn drop(&mut self) {
        println!("drop: {}", self.msg);
    }
}

pub fn main() {
    let c = {
        let one = Dropper::new("one");
        let two = Dropper::new("two");
        let three = Dropper::new("three");
        |:| { let _ = (&one, &two, &three); }
    };
    println!("Dropping unboxed closure");
    drop(c);
}

Without the patch, borrowck claims that the closure's capture of its upvars will outlive the upvars.

[alexcrichton]

ping @pcwalton, any update on this?