Add codegen support for __null

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@61314 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/CodeGenCXX/__null.cpp b/test/CodeGenCXX/__null.cpp
new file mode 100644
index 0000000..29416ce
--- /dev/null
+++ b/test/CodeGenCXX/__null.cpp
@@ -0,0 +1,9 @@
+// RUN: clang %s -emit-llvm -o %t
+
+int* a = __null;
+int b = __null;
+
+void f() {
+  int* c = __null;
+  int d = __null;
+}