Objective-C ARC. Allow conversion of (void*) pointers to
retainable ObjC pointers without requiring a bridge-cast
in the context of pointer comparison as this is in effect 
a +0 context. // rdar://16627903


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211243 91177308-0d34-0410-b5e6-96231b3b80d8