Driver/Darwin: Darwin uses -fobjc-legacy-dispatch on ARM.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@95006 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index 88dffa9..b0278b8 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -1031,7 +1031,7 @@
       // defaults to off.
       if (Args.hasFlag(options::OPT_fobjc_legacy_dispatch,
                        options::OPT_fno_objc_legacy_dispatch,
-                       false))
+                       getToolChain().IsObjCLegacyDispatchDefault()))
         CmdArgs.push_back("-fobjc-legacy-dispatch");
     }
   }