swift234

If a function conversion fails, suggest wrapping in a closure.

And even if we don't suggest wrapping in a closure (say, because there's
already a closure involved), emit a more relevant diagnostic anyway.
(Wordsmithing welcome.)

Wrapping a function value in a closure essentially explicitly inserts a
conversion thunk that we should eventually be able to implicitly insert;
that's rdar://problem/19517003.

Part of rdar://problem/19600325

Swift SVN r24987