Fix another place where clang check objc selector name instead of checking the selector family

Summary: In ARC mode, clang emits a warning if the result of an 'init' method is unused but miss cases where the method does not follows the Cocoa naming convention but is properly declared as an init family method.

CC: cfe-commits, eli.friedman

Differential Revision: http://llvm-reviews.chandlerc.com/D1163