Teach libc++ about the addressof() overloads it needs to work with

Objective-C Automatic Reference Counting, where Objective-C object
pointers can have several different qualifiers (__strong, __weak,
__autoreleasing, __unsafe_unretained). These addressof() overloads are
only provided in ARC mode, and the __weak variant is conditionalized
on having weak-reference support in the ARC runtime.

For historical reasons, Clang provides these definitions itself, and
defines the macro _LIBCPP_PREDEFINED_OBJC_ARC_ADDRESSOF to note when
it as done so. The code belongs here, and this redundancy will be
eliminated in the future.

Addresses <rdar://problem/9658274>.

llvm-svn: 133656