Index: lib/AST/TemplateBase.cpp
===================================================================
--- lib/AST/TemplateBase.cpp
+++ lib/AST/TemplateBase.cpp
@@ -62,6 +62,12 @@
     Out << "'";
   } else {
     Out << Val;
+    // Handle cases where the value is too large to fit into the underlying type
+    // i.e. where the unsignedness matters.
+    if (const BuiltinType *BT = T->getAs<BuiltinType>()) {
+      if (Val.isUnsigned() && Val.getBitWidth() == 64 && Val.isNegative())
+        Out << "ull";
+    }
   }
 }