[libc++] Add the <__assertion_handler> header

Because of llvm/llvm-project#77883, libc++
headers now #include an __assertion_handler header file that is
either generated by CMake or provided by the vendor. Since we don't use
CMake for libc++, this CL vendors <__assertion_handler> by copying the
file default_assertion_handler.in [0], which is the file that CMake uses
to generate the default version of <__assertion_handler>.

This CL is needed to unblock the libc++ autoroller (since otherwise
libc++ won't compile due to the missing header).

[0]: https://github.com/llvm/llvm-project/blob/main/libcxx/vendor/llvm/default_assertion_handler.in

Bug: 1517992
Change-Id: Ia551bdb706f7e68db914ddf46bf2128cdb1a6284
Fixed: 1517992
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5208502
Reviewed-by: Hans Wennborg <[email protected]>
Commit-Queue: Alan Zhao <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1248897}
NOKEYCHECK=True
GitOrigin-RevId: 0f6a1d2429de239f87ba3f199c14631d754acd1e

Author: alanzhao1

third_party/libc++/BUILD.gn

@@ -69,7 +69,10 @@ target(_libcxx_target_type, "libc++") {
     libs = [ "atomic" ]
   }
 
-  inputs = [ "__config_site" ]
+  inputs = [
+    "__assertion_handler",
+    "__config_site",
+  ]
 
   # TODO(crbug.com/1458042): Move this build file to third_party/libc++/BUILD.gn once submodule migration is done
   sources = [

third_party/libc++/__assertion_handler

@@ -0,0 +1,23 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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 _LIBCPP___ASSERTION_HANDLER
+#define _LIBCPP___ASSERTION_HANDLER
+
+#include <__config>
+#include <__verbose_abort>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+// TODO(hardening): in production, trap rather than abort.
+#define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_ABORT("%s", message)
+
+#endif // _LIBCPP___ASSERTION_HANDLER