OperationQueue.main dangling pointer fix

[bubski]

Calling OperationQueue.main would
instantiate an OperationQueue,
set an Unmanaged reference to it as main queue specific
and return OperationQueue.

Subsequent calls to OperationQueue.main would
get the Unmanaged reference to OperationQueue from main queue specific
and return OperationQueue wrapped in Unmanaged.

However, there's nothing stopping the OperationQueue from being deallocated, so the Unmanaged reference from subsequent call would in fact return garbage.

[parkera]

@swift-ci test and merge