[Sema][ObjC] Fix a -Wformat false positive with localizedStringForKey

Only honour format_arg attributes on -[NSBundle localizedStringForKey] when its
argument has a format specifier in it, otherwise its likely to just be a key to
fetch localized strings.

Fixes rdar://23622446

Differential revision: https://reviews.llvm.org/D27165

llvm-svn: 368878