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
