[ARC] When casting from a pointer to an objective-c object with known ownership,...

[ARC] When casting from a pointer to an objective-c object with known ownership, if the
cast type has no ownership specified, implicitly "transfer" the ownership of the cast'ed type
to the cast type:

id x;
(NSString**)&x; // Casting as (__strong NSString**).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@134275 91177308-0d34-0410-b5e6-96231b3b80d8