These implement DR resolutions and fix bugs.

    Fix error handling in filesystem::is_empty
    
        * src/filesystem/ops.cc (is_empty): Fix error handling.
        * testsuite/experimental/filesystem/operations/is_empty.cc: New test.

    PR71337 fix filesystem::temp_directory_path error handling
    
        PR libstdc++/71337
        * src/filesystem/ops.cc (temp_directory_path): Pass error_code
        argument to other filesystem operations.
        * testsuite/experimental/filesystem/operations/temp_directory_path.cc:
        Add testcase for inaccessible directory.

    Make directory iterators become end iterator on error
    
        * src/filesystem/dir.cc (open_dir): Return same value for errors
        whether ignored or not.
        (_Dir::advance(error_code*, directory_options)): Return false on
        error.
        (directory_iterator(const path&, directory_options, error_code*)):
        Create end iterator on error (LWG 2723).
        (recursive_directory_iterator(const path&, directory_options,
        error_code*)): Likewise.
        * testsuite/experimental/filesystem/iterators/directory_iterator.cc:
        Update expected behaviour on error.
        * testsuite/experimental/filesystem/iterators/
        recursive_directory_iterator.cc: Likewise.

    Do not retry failed close(3) in filesystem::copy
    
        * src/filesystem/ops.cc (close_fd): Remove.
        (do_copy_file): Just use close(3) instead of close_fd, to prevent
        retrying on error.

    Implement DR resolutions for filesystem::copy
    
        * src/filesystem/ops.cc (do_copy_file): Return an error if either
        source or destination is not a regular file.
        (copy): Update comment to refer to LWG 2681. Implement 2682 and 2683
        resolutions.
        (read_symlink): Add missing ec.clear().
        * testsuite/experimental/filesystem/operations/copy.cc: Update
        expected behaviour for copying directories with create_symlinks.
        Verify that error_code arguments are cleared if there's no error.
        * testsuite/experimental/filesystem/operations/read_symlink.cc:
        * New.

Tested x86_64-linux, commiitted to trunk. I'm going to do some mass
backports for all filesystem fixes to the branches this week as well.