libcxx: update to LLVM 18

release/18.x branch, commit 78b99c73ee4b96fe9ce0e294d4632326afb2db42

This adds the flag `-D_LIBCPP_HARDENING_MODE` which is determined based
on the Zig optimization mode.

This commit also fixes libunwind, libcxx, and libcxxabi to properly
report sub compilation errors.

lib/libcxx/src/ostream.cpp

@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include <__availability>
+#include <__config>
+#ifndef _LIBCPP_HAS_NO_FILESYSTEM
+#  include <fstream>
+#endif
+#include <ostream>
+
+#include "std_stream.h"
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+_LIBCPP_EXPORTED_FROM_ABI FILE* __get_ostream_file(ostream& __os) {
+  // dynamic_cast requires RTTI, this only affects users whose vendor builds
+  // the dylib with RTTI disabled. It does not affect users who build with RTTI
+  // disabled but use a dylib where the RTTI is enabled.
+  //
+  // Returning a nullptr means the stream is not considered a terminal and the
+  // special terminal handling is not done. The terminal handling is mainly of
+  // importance on Windows.
+#ifndef _LIBCPP_HAS_NO_RTTI
+  auto* __rdbuf = __os.rdbuf();
+#  ifndef _LIBCPP_HAS_NO_FILESYSTEM
+  if (auto* __buffer = dynamic_cast<filebuf*>(__rdbuf))
+    return __buffer->__file_;
+#  endif
+
+  if (auto* __buffer = dynamic_cast<__stdoutbuf<char>*>(__rdbuf))
+    return __buffer->__file_;
+#endif // _LIBCPP_HAS_NO_RTTI
+
+  return nullptr;
+}
+
+_LIBCPP_END_NAMESPACE_STD