libstdc++: Improve directory iterator abstractions for openat

Currently the _Dir::open_subdir function decides whether to construct a
_Dir_base with just a pathname, or a file descriptor and pathname. But
that means it is tiughtly coupled to the implementation of
_Dir_base::openat, which is what actually decides whether to use a file
descriptor or not. If the derived class passes a file descriptor and
filename, but the base class expects a full path and ignores the file
descriptor, then recursive_directory_iterator cannot recurse.

This change introduces a new type that provides the union of all the
information available to the derived class (the full pathname, as well
as a file descriptor for a directory and another pathname relative to
that directory). This allows the derived class to be agnostic to how the
base class will use that information.

libstdc++-v3/ChangeLog:

	* src/c++17/fs_dir.cc (_Dir::dir_and_pathname):: Replace with
	current() returning _At_path.
	(_Dir::_Dir, _Dir::open_subdir, _Dir::do_unlink): Adjust.
	* src/filesystem/dir-common.h (_Dir_base::_At_path): New class.
	(_Dir_base::_Dir_Base, _Dir_base::openat): Use _At_path.
	* src/filesystem/dir.cc (_Dir::dir_and_pathname): Replace with
	current() returning _At_path.
	(_Dir::_Dir, _Dir::open_subdir): Adjust.

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