Fix param forwarding
Fix param forwarding

Parameter forwarders cannot stay private, since private members
are not allowed to have the same name as inherited public ones.

Review by @DarkDimius

lampepfl/dotty,lampepfl/dotty,yusuke2255/dotty,svalaskevicius/dotty,densh/dotty,VladimirNik/dotty,som-snytt/dotty,reactormonk/dotty,dotty-staging/dotty,auvik/dotty,sjrd/dotty,lampepfl/dotty,som-snytt/dotty,lampepfl/dotty,yusuke2255/dotty,sjrd/dotty,reactormonk/dotty,sjrd/dotty,sjrd/dotty,densh/dotty,som-snytt/dotty,auvik/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,som-snytt/dotty,som-snytt/dotty,svalaskevicius/dotty,VladimirNik/dotty,dotty-staging/dotty