Closes #474

Overview:

• comment_q and annotation_comments contain all comments, i.e. regular Haddock comments (which are also present in the AST via HsDocString), invalid Haddock comments (which are not present in the AST) and regular, non-Haddock comments.
• We now print the invalid Haddock comments as regular comments, where we have to be careful:
  • Multiline invalid Haddock comments are not parsed as multiple single-line comments (in contrast to regular comments). This requires some changes in mkComment and AST diffing.
  • We have to insert a space after the hyphens if the comment does not start with a space to prevent a weird issue (see invalid-haddock-weird.hs).