[C2y] Add stdcountof.h

WG14 N3469 changed _Lengthof to _Countof but it also introduced the
<stdcountof.h> header to expose a macro with a non-ugly identifier.
GCC vends this header as part of the compiler implementation, so Clang
should do the same.

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -249,7 +249,9 @@ C2y Feature Support
   a conforming extension in earlier C language modes, but not in C++ language
   modes (``std::extent`` and ``std::size`` already provide the same
   functionality but with more granularity). The feature can be tested via
-  ``__has_feature(c_countof)`` or ``__has_extension(c_countof)``.
+  ``__has_feature(c_countof)`` or ``__has_extension(c_countof)``. This also
+  adds the ``<stdcountof.h>`` header file which exposes the ``countof`` macro
+  which expands to ``_Countof``.
 
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/lib/Headers/stdcountof.h

@@ -0,0 +1,15 @@
+/*===---- stdcountof.h - Standard header for countof -----------------------===
+ *
+ * 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
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef __STDCOUNTOF_H
+#define __STDCOUNTOF_H
+
+#define countof _Countof
+
+#endif /* __STDCOUNTOF_H */

clang/test/C/C2y/n3469.c

@@ -1,9 +1,9 @@
-// RUN: %clang_cc1 -fsyntax-only -std=c2y -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c2y -verify -ffreestanding %s
 
 /* WG14 N3469: Clang 21
  * The Big Array Size Survey
  *
- * This renames _Lengthof to _Countof.
+ * This renames _Lengthof to _Countof and introduces the stdcountof.h header.
  */
 
 void test() {
@@ -12,3 +12,12 @@ void test() {
                                expected-error {{expected expression}}
 }
 
+#ifdef countof
+#error "why is countof defined as a macro?"
+#endif
+
+#include <stdcountof.h>
+
+#ifndef countof
+#error "why is countof not defined as a macro?"
+#endif