bug: udvts of shielded types can be returned from public/external functions

[samlaf]

In #114 we are making any type that contains a shielded type impossible to be returned from a public/external function. The goal here is to prevent silly mistakes and force devs to type-cast to fully public types when returning values.

However, I have noticed that our solidity fork does not consider user defined value types (udvts). For example this test fails:

// test/libsolidity/syntaxTests/visibility/shielded_udvt_return.sol
type mysbool is sbool;

contract Deployer {
    mysbool private storedBoolean;

    // SHOULD RETURN AN ERROR, BUT DOESN'T
    function getStoredBooleanFailExternal() external returns (mysbool) {
        return storedBoolean;
    }

    // SHOULD RETURN AN ERROR, BUT DOESN'T
    function getStoredBooleanFailPublic() public returns (mysbool) {
        return storedBoolean;
    }
}
// ----
// DeclarationError 7492: (217-222): Shielded objects cannot be returned from public or external functions. Use internal or private functions or cast to an unshielded type.
// DeclarationError 7492: (321-326): Shielded objects cannot be returned from public or external functions. Use internal or private functions or cast to an unshielded type.

[cdrappi]

205508d