[red-knot] fix handling of known instances in `Type` relation methods

[carljm]

We currently represent "known instances" (e.g. special forms like typing.Literal, which are an instance of typing._SpecialForm, but need to be handled differently from other instances of typing._SpecialForm) as an InstanceType with a known field that is Some(...).

This makes it easy to handle a known instance as if it were a regular instance type (by ignoring the known field), and in some cases (e.g. Type::member) that is correct and convenient. But in other cases (e.g. Type::is_equivalent_to) it is not correct, and we currently have a bug that we would consider the known-instance type of typing.Literal as equivalent to the general instance type for typing._SpecialForm, and we would fail to consider it a singleton type or a single-valued type (even though it is both.)

An instance type with known.is_some() is semantically quite different from an instance type with known.is_none(). The former is a singleton type that represents exactly one runtime object; the latter is an open type that represents many runtime objects, including instances of unknown subclasses. It is too error-prone to represent these very-different types as a single Type variant. We should instead introduce a dedicated Type::KnownInstance variant and force ourselves to handle these explicitly in all Type variant matches.