Objective-C ARC. Allow conversion of (void*) pointers to
retainable ObjC pointers without requiring a bridge-cast
by recognizing this as a +0 context. // rdar://16627903


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211234 91177308-0d34-0410-b5e6-96231b3b80d8