libstdc++: Implement LWG 3070 in path::lexically_relative

libstdc++: Implement LWG 3070 in path::lexically_relative

	* src/c++17/fs_path.cc [_GLIBCXX_FILESYSTEM_IS_WINDOWS]
	(is_disk_designator): New helper function.
	(path::_Parser::root_path()): Use is_disk_designator.
	(path::lexically_relative(const path&)): Implement resolution of
	LWG 3070.
	* testsuite/27_io/filesystem/path/generation/relative.cc: Check with
	path components that look like a root-name.

From-SVN: r278313