Refine merge of nullary and parameterless denotations.
Refine merge of nullary and parameterless denotations.

Previously, two denotations with types => T and ()T could not
be merged, only their types could be, but then the denotation
would no longer be a SymDenotation. We now treat the two types
as equivalent so that it will select the symbol in a subclass.
This fixes a problem where, once overrding pairs are made to
work (see following commit), in core/Types we get an error "overriding
final method "hashCode".

dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,spetz911/dotty,angi2/dotty,som-snytt/dotty,AlexSikia/dotty,som-snytt/dotty,densh/dotty,yusuke2255/dotty,lampepfl/dotty,auvik/dotty,lampepfl/dotty,svalaskevicius/dotty,angi2/dotty,sjrd/dotty,sjrd/dotty,som-snytt/dotty,yusuke2255/dotty,reactormonk/dotty,vsalvis/dotty,svalaskevicius/dotty,reactormonk/dotty,VladimirNik/dotty,dotty-staging/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty,spetz911/dotty,AlexSikia/dotty,vsalvis/dotty,lampepfl/dotty,folone/dotty,som-snytt/dotty,lampepfl/dotty,densh/dotty,folone/dotty,dotty-staging/dotty,dotty-staging/dotty,VladimirNik/dotty,auvik/dotty,som-snytt/dotty