In #1040 I had convinced myself that the type ignore logic didn't
need anything like the ignored_ids from the type comment logic, but I
was wrong, and we do.
We hit these cases in practice a bunch.
Type comments only apply if they are the first comment on the line,
which means that allowing them to be pushed behind a regular comment
when joining lines is a semantic change (and, indeed, one that black
catches and fails on).
