[clang] Ensure proper NULL macro definition for system include files. (PR #149176)

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,24 @@
+/*===---- dbm.h - BSD header for database management ----------------------===*\
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+\*===----------------------------------------------------------------------===*/
+
+#if !defined(_AIX)
+
+#include_next <dbm.h>
+
+#else
+
+#define __need_NULL
+#include <stddef.h>
+
+#include_next <dbm.h>
+
+/* Ensure that the definition of NULL is as expected. */
+#define __need_NULL
+#include <stddef.h>
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AaronBallman wrote:

Why is the pattern to define `NULL`, include_next the header the user asked for, and then define `NULL` again? Are the include_next headers defining `NULL` to something different? And if so, what are we potentially breaking by redefining it out from under them?

https://github.com/llvm/llvm-project/pull/149176