Implicitly annotate __CFStringMakeConstantString with format_arg(1).

We handled the builtin version of this function in r157968, but the builtin
isn't used when compiling as -fno-constant-cfstrings.

This should complete <rdar://problem/6157200>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161525 91177308-0d34-0410-b5e6-96231b3b80d8