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.
The code introduced in #1027 to detect whether a type comment appeared
after a regular comment in a Line would spuriously misfire when a leaf
was in the comments dict but had an empty list of comments. This can
occur as an artifact of how comments on trailing commas are handled,
it seems.
(This was discovered trying to test black out on mypy.)
