Use decltype instead, to prevent MSVC 2017.5 warnings.

Change-Id: I760659837e34eb2dc2a3293c43e7f29725b515d6
Reviewed-on: https://gerrit.libreoffice.org/48221Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>