We kept this around for a while since Xcode 6 and earlier had a build
setting for this warning. It was removed in Xcode 7 so there should be
no need for this warning now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@266938 91177308-0d34-0410-b5e6-96231b3b80d8