Don't consider contracts implicitly convertible to the type of super

- `super` for all intents and purposes behaves like a type and should be a TypeType rather than ContractType. We have an issue to fix it but it's a breaking change. Until then let's at least not treat other contracts as convertible to the ContractType that `super` represents.

libsolidity/ast/Types.cpp

@@ -1569,11 +1569,15 @@ BoolResult ContractType::isImplicitlyConvertibleTo(Type const& _convertTo) const
 		return true;
 	if (_convertTo.category() == Category::Contract)
 	{
+		auto const& targetContractType = dynamic_cast<ContractType const&>(_convertTo);
+		if (targetContractType.isSuper())
+			return false;
+
 		auto const& bases = contractDefinition().annotation().linearizedBaseContracts;
 		return find(
 			bases.begin(),
 			bases.end(),
-			&dynamic_cast<ContractType const&>(_convertTo).contractDefinition()
+			&targetContractType.contractDefinition()
 		) != bases.end();
 	}
 	return false;