Add a new Ignored attribute type, and use it for may_alias.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64446 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaDeclAttr.cpp b/lib/Sema/SemaDeclAttr.cpp
index 833a52b..b585bd5 100644
--- a/lib/Sema/SemaDeclAttr.cpp
+++ b/lib/Sema/SemaDeclAttr.cpp
@@ -1372,6 +1372,9 @@
   case AttributeList::AT_pure:        HandlePureAttr      (D, Attr, S); break;
   case AttributeList::AT_cleanup:     HandleCleanupAttr   (D, Attr, S); break;
   case AttributeList::AT_nodebug:     HandleNodebugAttr   (D, Attr, S); break;
+  case AttributeList::IgnoredAttribute: 
+    // Just ignore
+    break;
   default:
 #if 0
     // TODO: when we have the full set of attributes, warn about unknown ones.