When I leave a comment that allows the OP to resolve the problem, great, but that wasn't my intention.
I usually do this when I don't have the time (or simply don't want) to write a proper answer. I still want to leave the hint for the author of the question.
Now when the author follows that hint, it was actually useful and it allowed the author to resolve the issue, then there are several possible outcomes:
Someone else already posted an answer containing the same or similar enough solution so that that answer can be accepted. (This also includes the possibility of someone blatantly copying the contents of my comment). I won't mind, because if I wanted to have the accepted answer on the question, I should have posted a proper answer in the first place.
The author leaves me a comment, notifying me that my comment could actually be made into an answer, because it is the solution to the authors problem. Now there are again 2 possible outcomes:
I post an answer and everything is wonderful.
I neglect to post an answer. (This is also the case when the author doesn't notify me)
This case should evidently end up with the case stated under 1., someone else should simply copy the information. Here we make 2 further distinctions:
The someone is not the author of the original question.
In this case, it would be nice to be referenced in the answer, if the comment was the source of the information. Additionally, I would prefer the answer to be marked as Community Wiki, but this is really just a sign of courtesy (as in, I don't want to accept the possible reputation for this answer).
The someone is the author of the original question.
This is probably my favorite solution. This would simply be the "self-answer" case. The user found the solution on their own. Sure, the comment might have been the seed of the solution, but that shouldn't matter. The user should then formulate a proper answer and optionally reference the comment.
Comments are second-class citizens and if someone doesn't want to put in the effort to post a proper answer, we surely won't do the work for them in promoting their comment to a proper post. I feel like there are already enough possible ways to deal with a situation like that.
That being said, what I posted above is how I see the situation in regards to my own comments. If someone was to flag a comment telling me "This should be an answer", I would dismiss the flag.
I don't have a clear policy on answers that were sourced from other users comments. But it's somewhat along the lines of "Post your own answer or STFU".
somethinguseful to the post. Whether a comment is an answer or not does not really transcend the system.