treat enums as never members of unknown types

[bakkot]

The type lattice is deliberately conservative; when it encounters a type name it doesn't recognize, like PrivateElement, it assumes that could hold anything and as such it is never an error to assign any value to something of that type.

This heuristic is generally necessary; e.g. a property key is a String or a Symbol, and "property key" is an unknown type, so it is good that ecmarkup does not regard it as an error to assign a String to an unknown type.

However, we don't and (I weakly believe) shouldn't define any types that hold enums. This means we can assume a type name we don't recognize cannot hold an enum, and so assigning an enum specifically to such a type is an error.