Added a new attribute, objc_root_class, which informs the compiler when a root class is intentionally declared.
The warning this inhibits, -Wobjc-root-class, is opt-in for now. However, all clang unit tests that would trigger
the warning have been updated to use -Wno-objc-root-class. <rdar://problem/7446698>


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154187 91177308-0d34-0410-b5e6-96231b3b80d8