A `@convention(block)` closure in Swift is completely compatible with Objective-C
and does not need to be wrapped in a `__SwiftValue` box for use.
Previously, it was bridged verbatim when appearing by itself, but
could end up boxed when it went through array bridging.
The test verifies that:
* Objective-C does not see a `__SwiftValue` box
* Swift `type(of:)` does not see a `__SwiftValue` box
* Objective-C can actually call the closure
Resolves rdar://138132321
