objective-C: clang, following gcc, warns on
use of stand-alone protocol as type and uses
id<proto>. Modify warning to say what compiler 
is doing. // rdar//13158394


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176303 91177308-0d34-0410-b5e6-96231b3b80d8