Tweak diagnostic text to indicate that __weak on a local variable is only allowed
for ARC.  Fixes <rdar://problem/12407705>

llvm-svn: 164990
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 4382432..ddb41ed 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -4578,8 +4578,10 @@
       && !NewVD->hasAttr<BlocksAttr>()) {
     if (getLangOpts().getGC() != LangOptions::NonGC)
       Diag(NewVD->getLocation(), diag::warn_gc_attribute_weak_on_local);
-    else
+    else {
+      assert(!getLangOpts().ObjCAutoRefCount);
       Diag(NewVD->getLocation(), diag::warn_attribute_weak_on_local);
+    }
   }
   
   bool isVM = T->isVariablyModifiedType();