Re-add + update __assertion_handler from default_assertion_handler.in

This file was added as a part of LLVM 18 update (#21638) in
8d51927
and mistakenly deleted when I ran update_libcxx.py. This file was copied
from
https://github.com/llvm/llvm-project/blob/aadaa00de76ed0c4987b97450dd638f63a385bed/libcxx/vendor/llvm/default_assertion_handler.in, so this also updates the file with the newest `default_assertion_handler.in`.

Author: aheejin

system/lib/libcxx/include/__assertion_handler

@@ -0,0 +1,41 @@
+// -*- 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
+
+#if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
+
+#  define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_ABORT("%s", message)
+
+#else
+
+#  if __has_builtin(__builtin_verbose_trap)
+// AppleClang shipped a slightly different version of __builtin_verbose_trap from the upstream
+// version before upstream Clang actually got the builtin.
+// TODO: Remove once AppleClang supports the two-arguments version of the builtin.
+#    if defined(_LIBCPP_APPLE_CLANG_VER) && _LIBCPP_APPLE_CLANG_VER < 1700
+#      define _LIBCPP_ASSERTION_HANDLER(message) __builtin_verbose_trap(message)
+#    else
+#      define _LIBCPP_ASSERTION_HANDLER(message) __builtin_verbose_trap("libc++", message)
+#    endif
+#  else
+#    define _LIBCPP_ASSERTION_HANDLER(message) ((void)message, __builtin_trap())
+#  endif
+
+#endif // _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
+
+#endif // _LIBCPP___ASSERTION_HANDLER