objective-C arc: load of a __weak object happens via call to

objective-C arc: load of a __weak object happens via call to
objc_loadWeak. This retains and autorelease the weakly-refereced
object. This hidden autorelease sometimes makes __weak variable alive even
after the weak reference is erased, because the object is still referenced
by an autorelease pool. This patch overcomes this behavior by loading a
weak object via call to objc_loadWeakRetained(), followng it by objc_release
at appropriate place, thereby removing the hidden autorelease. // rdar://10849570

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168740 91177308-0d34-0410-b5e6-96231b3b80d8