Generalize rules for isSubType between NamedTypes.
Generalize rules for isSubType between NamedTypes.

Can assume P#T <: Q#T if P <: Q. This follows from
the rules how we expand # to existentials.

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