librustc: Tie up loose ends in unboxed closures.

[pcwalton]

This patch primarily does two things: (1) it prevents lifetimes from
leaking out of unboxed closures; (2) it allows unboxed closure type
notation, call notation, and construction notation to construct closures
matching any of the three traits.

This breaks code that looked like:

let mut f;
{
    let x = &5i;
    f = |&mut:| *x + 10;
}

Change this code to avoid having a reference escape. For example:

{
    let x = &5i;
    let mut f; // <-- move here to avoid dangling reference
    f = |&mut:| *x + 10;
}

I believe this is enough to consider unboxed closures essentially
implemented. Further issues (for example, higher-rank lifetimes) should
be filed as followups.

Closes #14449.

[breaking-change]

r? @pnkfelix

[sfackler]

Needs a rebase