[libc++] Add __assertion_handler as forwarding header

After #131031, Clang modules build
fails in chromium as we use libcxx directory directly like below.

```
stderr:
../../third_party/libc++/src/include/module.modulemap:2325:12: error: header '__assertion_handler' not found
 2325 |     header "__assertion_handler" // generated via CMake
      |            ^
1 error generated.
```

To fix this error, I added `__assertion_handler` as a forward header for our usage.

Author: atetubou

libcxx/include/__assertion_handler

@@ -0,0 +1,19 @@
+// -*- 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_next <__assertion_handler> // Note: this include is generated by CMake and is potentially vendor-provided.
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#endif // _LIBCPP___ASSERTION_HANDLER