rustc: Allow `return` to return from a closure.

With the old `for` gone, this behaviour is no longer conflicting with
that use of `return` in closures, and this allows shortcircuiting in a
closure.
auto merge of #11024 : huonw/rust/return-from-closures, r=alexcrichton

With the old `for` gone, this behaviour is no longer conflicting with
that use of `return` in closures, and this allows shortcircuiting in a
closure.