Objective-C++ ARC: Improve the conversion to a const __unsafe_unretained reference.

Under ARC++, a reference to a const Objective-C pointer is implicitly
treated as __unsafe_unretained, and can be initialized with (e.g.) a
__strong lvalue. Make sure this behavior does not break template
argument deduction and (related) that partial ordering still prefers a
'T* const&' template over a 'T const&' template when this case kicks
in. Fixes <rdar://problem/14467941>.