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/Parse/AttributeList.cpp b/lib/Parse/AttributeList.cpp
index 71712b6..7f18a2c 100644
--- a/lib/Parse/AttributeList.cpp
+++ b/lib/Parse/AttributeList.cpp
@@ -89,6 +89,7 @@
   case 9:
     if (!memcmp(Str, "dllimport", 9)) return AT_dllimport;
     if (!memcmp(Str, "dllexport", 9)) return AT_dllexport;
+    if (!memcmp(Str, "may_alias", 9)) return IgnoredAttribute;
     break;
   case 10:
     if (!memcmp(Str, "deprecated", 10)) return AT_deprecated;