Comment 2 for bug 2065904

Actually there's a slight issue with my implementation, as there is behavior that relies on being able to stop .sourcepos and .sourceline from being set so you can easily search for tags with those names. I'm sure I did it this way for backwards compatibility reasons, but I don't think anyone was actually using it this wayu, and the previous implementation had a couple big problems. First, the same code can do drastically different things in different contexts. Second, Tag.sourcepos and Tag.sourceline don't have a consistent data type, which is an issue as of 4.13. So I'm leaning towards enforcing the behavior I just added.