Exclude attributes from precondition visibility verification

[SergeyTeplyakov]

Fix for #273

[hubuk]

Current code checks declaringType.BaseType twice on recurrence.
Loop version should be simpler:

private static bool IsAttribute(TypeNode declaringType)
{
    TypeNode currentType = declaringType;
    while (currentType != null)
    {
        if (currentType == systemAttributeType.Value)
        {
            return true;
        }

        currentType = declaringType.BaseType;
    }

    return false;
}

[AlexanderTaeschner]

In the loop version instead of

currentType = declaringType.BaseType

this should be

currentType = currentType.BaseType

[hubuk]

You are right, thanks. Otherwise it might be less efficient than recursive version...