librustc: Make sure region bounds on closures outlive calls to them.

[pcwalton]

This can break code like:

fn call_rec(f: |uint| -> uint) -> uint {
    (|x| f(x))(call_rec(f))
}

Change this code to use a temporary instead of violating the borrow
rules:

fn call_rec(f: |uint| -> uint) -> uint {
    let tmp = call_rec(|x| f(x)); f(tmp)
}

Closes #17144.

[breaking-change]

r? @huonw