improve the detection of stuff we can return by const &, instead of by
copying

Change-Id: I479ae89d0413125a8295cc3cddbc0017ed61ed69
Reviewed-on: https://gerrit.libreoffice.org/46915Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>