PR libstdc++/87116 fix path::lexically_normal() handling of dot-dot

PR libstdc++/87116 fix path::lexically_normal() handling of dot-dot

Previously the logic that turned "a/b/c/../.." into "a/" failed to
preserve an empty path at the end of the iteration sequence, as required
by the trailing slash. That meant the result didn't meet the class
invariants, and that "a/b/c/d/../../.." would remove four components
instead of the three that "../../.." should remove.

	PR libstdc++/87116
	* src/filesystem/std-path.cc (path::lexically_normal): When handling
	a dot-dot filename, preserve an empty final component in the iteration
	sequence.
	[_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Use preferred-separator for
	root-directory.
	* testsuite/27_io/filesystem/path/generation/normal.cc: Add new tests
	for more than two adjacent dot-dot filenames.
	[_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Replace slashes with
	preferred-separator in expected normalized strings.

From-SVN: r263922