Objective-C. Under a special flag, -Wcstring-format-directive,
off by default, issue a warning if %s directive is used
in formart argument of a function/method declared as
__attribute__((format(CF/NSString, ...)))
To complete rdar://18182443


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217619 91177308-0d34-0410-b5e6-96231b3b80d8