add a hack for visual studio, fixing PR7796

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@110161 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Headers/stddef.h

@@ -40,6 +40,7 @@ typedef __typeof__(*L"") wchar_t;
 
 #undef NULL
 #ifdef __cplusplus
+#undef __null  // VC++ hack.
 #define NULL __null
 #else
 #define NULL ((void*)0)