GNUNullExpr is a valid sentinel even though it isn't of pointer type.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@89778 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/SemaCXX/attr-sentinel.cpp b/test/SemaCXX/attr-sentinel.cpp
new file mode 100644
index 0000000..0293a5d
--- /dev/null
+++ b/test/SemaCXX/attr-sentinel.cpp
@@ -0,0 +1,6 @@
+// RUN: clang-cc -fsyntax-only -verify %s
+void f(int, ...) __attribute__((sentinel));
+
+void g() {
+  f(1, 2, __null);
+}