Fix signature caching in NamedType
Fix signature caching in NamedType

Underdefined signatures cannot be cached, this matches the logic in
MethodicType.

I haven't managed to create a testcase that fails without this, but it
should be the Right Thing to do anyway.

som-snytt/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,som-snytt/dotty,som-snytt/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,lampepfl/dotty,som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty