(rather than 'open')
This attribute was originally introduced for Swift classes exposed to
Objective-C, but if it /is/ used on a pure Objective-C class Swift
should respect it. There might be an actual reason, such as a hot code
path in ContrivedExampleKit checking against the class object's
address instead of using -isKindOfClass:.
For source compatibility, only enforce this in Swift 5 mode.
rdar://problem/33971529
