[libcxx] [lld] [llvm] [clang] [flang] [libc++][streams] P1759R6: Native handles and file streams (PR #76632)

[Mark de Wever via cfe-commits]

================
@@ -245,6 +267,18 @@ public:
 #  endif
   _LIBCPP_HIDE_FROM_ABI basic_filebuf* __open(int __fd, ios_base::openmode __mode);
   basic_filebuf* close();
+#  if _LIBCPP_STD_VER >= 26
+  _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept {
+    _LIBCPP_ASSERT_UNCATEGORIZED(this->is_open(), "File must be opened");
+#    if defined(_LIBCPP_WIN32API)
+    return __filebuf_windows_native_handle(__file_);
----------------
mordante wrote:

```suggestion
    return std::__filebuf_windows_native_handle(__file_);
```

https://github.com/llvm/llvm-project/pull/76632