add a hack for visual studio, fixing PR7796

llvm-svn: 110161
diff --git a/clang/lib/Headers/stddef.h b/clang/lib/Headers/stddef.h
index b1d0d52..fdd4815 100644
--- a/clang/lib/Headers/stddef.h
+++ b/clang/lib/Headers/stddef.h
@@ -40,6 +40,7 @@
 
 #undef NULL
 #ifdef __cplusplus
+#undef __null  // VC++ hack.
 #define NULL __null
 #else
 #define NULL ((void*)0)