swift234

[objc] Fix closure->block conversion to handle closures with context.

Previously, we were relying on objc_retain to retain the closure, but
closure contexts aren't actually id-compatible. Now we use a C++ RAII
object for Swift to manage the closure context retain count.

In the process, Joe and I discovered that the closure context was being
passed to the conversion shim at +0; that has now been fixed to be +1.

Swift SVN r3948