[CodeGen][ObjC] Don't try to retain a __unsafe_unretained ARC pointer

[CodeGen][ObjC] Don't try to retain a __unsafe_unretained ARC pointer
passed to __builtin_os_log_format to extend its lifetime to the end of
its enclosing block

Extend only lifetimes of pointers returned by function calls or message
sends instead. In the long term, we should lifetime-extend pointers in
more complex expressions and non-ARC objects (e.g., C++ temporaries)
too.

rdar://problem/61846261