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/fstream.cpp

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <__config>
+#include <cstdio>
+#include <fstream>
+
+#if defined(_LIBCPP_WIN32API)
+#  define WIN32_LEAN_AND_MEAN
+#  define NOMINMAX
+#  include <io.h>
+#  include <windows.h>
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if defined(_LIBCPP_WIN32API)
+
+// Confirm that `HANDLE` is `void*` as implemented in `basic_filebuf`
+static_assert(std::same_as<HANDLE, void*>);
+
+_LIBCPP_EXPORTED_FROM_ABI void* __filebuf_windows_native_handle(FILE* __file) noexcept {
+  // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=msvc-170
+  intptr_t __handle = _get_osfhandle(fileno(__file));
+  if (__handle == -1)
+    return nullptr;
+  return reinterpret_cast<void*>(__handle);
+}
+
+#endif
+
+_LIBCPP_END_NAMESPACE_STD