Underline type nodes without position mappings in .types baselines

[weswigham]

The underlines themselves, we've talked about in general already, but the underlining logic should get a proper look-over - I think it's pretty good, but I wrote it 😄

[jakebailey]

Underling code seems fine to me; I can't think of a better way to do it.

Of course, the rest of the change is HugeMcLarge and we're going to have to fix all PRs, but I think we were okay with it just to get this coverage.

[rbuckton]

Unfortunately, due to the number of baselines that changed there are too many files in the diff view and it's crashing the browser tab when I try to comment. I provided some feedback to @weswigham offline.

Otherwise, LGTM.