When creating a NamedType with a given overloaded denotation, make sure that
the type has a Name as designator. This prevents accidentally overwriting a more
precise symbolic TermRef that refers to one specific alternative of the denotation.

This might be enough to fix scala#16884.

It would be great if we could maintain the general invariant that NamedTypes with
overloaded denotations always have names as designators. But that looks very hard
when we take into account that we need to update named types to new runs.
A type might have a single denotation in one round and an overloaded one in the
next.

Make infoDependsOnPrefix return true even if the prefix is
the ThisType of the enclosing class, in case that class has a selftype
which refines the binding of an abstract type.

This allows to implement withDenot without overwriting a NamedType's state.

Also consider non-final term members with self type refinements for infoDependsOnPrefix