[clang-doc] Refactor `CommentInfo.Kind` from string to `CommentKind` enum in clang-doc

[snarang181]

We should refactor the clang-doc tool’s internal handling of comment kinds by migrating the CommentInfo.Kind field from a raw SmallString<16> (string) to a strongly-typed enum, CommentKind. This should make iteration of the different fields much more convenient.

[snarang181]

@ilovepi, I saw the issue you created #142083 and how it suggests the refactoring of the CommentInfo.Kind field. I started working on it yesterday and saw the diff is substantial so decided to break down the issues.

The draft PR I have up is linked to this issue.

[ilovepi]

This is a duplicate of #142083. I'm not sure what value a second issue covering the same problem achieves.