...instead of blacklisting such cases.  Reuses the
checkIdenticalDefaultArguments code that was originally in
loplugin:overrideparam (and appears to work reasonably well for the default
arguments that actually happen in practice).

Change-Id: I9cf2db17101beb135b2039a9b7ed335bd2af2c08
Reviewed-on: https://gerrit.libreoffice.org/44594Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>