Objective-C. Under a special flag, -Wcstring-format-directive,
off by default, issue a warning if %s directive is used in
certain CF/NS formatting APIs, to assist user in deprecating
use of such %s in these APIs. rdar://18182443


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217467 91177308-0d34-0410-b5e6-96231b3b80d8