Define NULL to __null in C++, so that it's guaranteed to have the same size as a pointer

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@60355 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Headers/stddef.h b/lib/Headers/stddef.h
index d5f4eb9..2c84b4b 100644
--- a/lib/Headers/stddef.h
+++ b/lib/Headers/stddef.h
@@ -33,7 +33,7 @@
 #endif
 
 #ifdef __cplusplus
-#define NULL (0)
+#define NULL __null
 #else
 #define NULL ((void*)0)
 #endif