[driver] Emit an error when trying to use ARC on macosx earlier than 10.6

rdar://10459258

llvm-svn: 151706
diff --git a/clang/lib/Driver/ToolChains.h b/clang/lib/Driver/ToolChains.h
index 0fb2138..0d59110 100644
--- a/clang/lib/Driver/ToolChains.h
+++ b/clang/lib/Driver/ToolChains.h
@@ -379,6 +379,8 @@
 
   virtual bool SupportsObjCGC() const;
 
+  virtual bool SupportsObjCARC() const;
+
   virtual bool UseDwarfDebugFlags() const;
 
   virtual bool UseSjLjExceptions() const;