swift234

Runtime: Add ObjC weak referencing support to SwiftObject.

We were missing -_tryRetain, -_isDeallocating, -allowsWeakReference and -retainWeakReference implementations on SwiftObject, so forming an ObjC weak reference to a pure Swift object always failed and produced a nil reference. Fixes rdar://problem/18637774.

This can be reapplied now that we properly call objc_destructInstance on deallocation.

Swift SVN r23070