[lib/Headers] Define NULL as __DARWIN_NULL when on __APPLE__.

This makes it identical with the system definition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178110 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Headers/stddef.h

@@ -53,7 +53,9 @@ typedef __WCHAR_TYPE__ wchar_t;
 #endif
 
 #undef NULL
-#ifdef __cplusplus
+#if defined(__APPLE__) && defined(__DARWIN_NULL)
+#  define NULL __DARWIN_NULL
+#elif defined(__cplusplus)
 #  if !defined(__MINGW32__) && !defined(_MSC_VER)
 #    define NULL __null
 #  else