Add in an unreachable after a covered switch to appease certain bots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232356 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/ObjCARC/ObjCARC.h

@@ -304,6 +304,7 @@ public:
             M->getContext().getMDKindID("clang.arc.no_objc_arc_exceptions");
       return *NoObjCARCExceptionsMDKind;
     }
+    llvm_unreachable("Covered switch isn't covered?!");
   }
 };