libcxx: Add __assertion_handler as forwarding header

Author: atetubou

libcxx/include/__assertion_handler

@@ -0,0 +1,15 @@
+// -*- 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.
+
+#endif // _LIBCPP___ASSERTION_HANDLER