[clang] [clang][headers] Including stddef.h always redefines NULL (PR #99727)

[Aaron Ballman via cfe-commits]

================
@@ -20,19 +20,18 @@
  * modules.
  */
 #if defined(__MVS__) && __has_include_next(<stdarg.h>)
-#include <__stdarg_header_macro.h>
 #undef __need___va_list
 #undef __need_va_list
 #undef __need_va_arg
 #undef __need___va_copy
 #undef __need_va_copy
+#include <__stdarg_header_macro.h>
----------------
AaronBallman wrote:

Can you add additional test coverage for the changes to this file?

https://github.com/llvm/llvm-project/pull/99727