There's no point flagging such comments. Flags are seen by moderators, not the comment author, and moderators don't have any special power to make comments into answers. I think it would be really useful to have a special kind of "this comment should be an answer" flag, which goes straight to the comment author, but we don't have such a thing.
The only sensible course of action is to reply to the comment to suggest that the comment is a helpful answer and if posted as an answer, it would be easier for people to find and that you would vote up that answer. I've emphasized three parts of that, for the following reason.
Remember that everyone here is acting voluntarily. We can't force anyone to write an answer, and there's no obligation for people to write full posts. People contribute because they want to help others, and for recognition from other people via the site's reputation system. It's important not to act like someone has short-changed you by making their suggestion in a comment instead of an answer, or like you're entitled to a full post.
For that reason, I think it's important to be honest that you're asking a favour of the author. The three important parts of the favour-asking are (a) showing that you appreciate what they've already done; (b) showing how writing an answer is even more helpful than what they've already done; and (c) showing how writing an answer will earn them recognition from the community.
I know it's hard to be as encouraging as a kindergarten teacher all of the time. Certainly on occasion when I've been having a bad day and I know that whatever comment I write will be mean and sarcastic instead of encouraging, I've simply ignored the answer-as-comment instead of trying to get the author to fix it. I'd encourage anyone on the site to do the same if they feel that writing a suggestion with my three points is too much work, or is draining, and if they feel tempted to write a rude message asking someone . There are many reasons why answers-in-comments are bad for the site, but none of those is worth alienating users for.
So although I said the only sensible course of action is to reply to the comment, your "do nothing" option can also be sensible sometimes. As a community we pride ourselves on picking up litter, but sometimes it's OK to just leave one for someone else to find.
