Fix handling of an empty filename at end of a path

Fix handling of an empty filename at end of a path

The C++17 std::filesystem::path grammar allows an empty filename as the
last component (to signify a trailing slash). The existing code does not
handle this consistently, sometimes an empty filename has type _Multi
and sometimes it has type _Filename. This can result in a non-empty
iterator range for an empty filename component.

This change ensures that empty paths always have type _Filename and will
yield an empty iterator range.

	* include/bits/fs_path.h (path::_M_type): Change default member
	initializer to _Filename.
	(path::begin): Create past-the-end iterator for empty path.
	* src/filesystem/std-path.cc (path::remove_filename()): Remove
	debugging check.
	(path::has_relative_path()): Return false for empty filenames.
	(path::_M_split_cmpts): Set _M_type to _Filename for empty paths.
	Fix offset of empty final component.
	* testsuite/27_io/filesystem/path/itr/components.cc: New.
	* testsuite/27_io/filesystem/path/itr/traversal.cc: Add new inputs.

From-SVN: r260616