SI-7109 SI-7153 Generalize the API to get docComments: allow to force docTrees
for given fragments. Don't type-check when forcing doc comments, but rather
 do it directly. Test the new functionality as well as better tests for
the old one.
Merge pull request #2156 from vigdorchik/hierarchy

SI-7109 SI-7153 Generalize the API to get docComments: allow to force do...