I don't think you should flag those - moderators don't have a "convert comment to answer" feature, so they can't act on your flag directly.
What you can do though is:
- Ask the person who answered in comments to make a proper answer – this is more likely to work for recent comments from active users obviously.
- Post the answer yourself if the above hasn't worked (or doesn't look like it could work). If you feel guilty about "stealing" the answer (and potential reputation points attached), mark your answer as a community wiki. (And mention that the solution was provided by @whoever too.)
Of course moderators could do that too, but that's not really their job.
In case you feel like summarizing a long comment thread into a proper answer, one thing you could flag is the comment thread itself since it has become obsolete (flag the first comment and explain what you did and that the whole thread could be cleaned up). That's double good: proper summarized answer and less comment noise.