PR libstdc++/83891 fix path::is_absolute() for non-POSIX targets

PR libstdc++/83891 fix path::is_absolute() for non-POSIX targets

The correct definition seems to be has_root_directory() for all systems
we care about.

	PR libstdc++/83891
	* include/bits/fs_path.h (path::is_absolute()): Use same definition
	for all operating systems.
	* include/experimental/bits/fs_path.h (path::is_absolute()): Likewise.
	* testsuite/27_io/filesystem/path/query/is_absolute.cc: New.
	* testsuite/27_io/filesystem/path/query/is_relative.cc: Fix comment.
	* testsuite/experimental/filesystem/path/query/is_absolute.cc: New.

From-SVN: r260259