RFR: JDK-8288624: Cleanup CommentHelper.getText0 [v4]

[Jonathan Gibbons]

> Please review a moderately simple fix to clean up (as in _remove_!) `CommentHelper.getText` and friends/overloads.
> 
> This is moderately simple, because most of the heavy lifting was done in 
> [JDK-8288699](https://bugs.openjdk.org/browse/JDK-8288624), to clean up `commentTagsToContent`.
> 
> The uses of `CommentHelper.getText` can generally be replaced by either `commentTagsToContent` or just `DocTree.toString()`.
> 
> Two bugs were uncovered as a result of the cleanup.  These are described in detail in a comment with screenshots in the [bug report](https://bugs.openjdk.org/browse/JDK-8288624?focusedCommentId=14508488&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14508488)  
> 
> Fixing the `see-list-long` bug was a direct reason to cleanup `commentTagsToContent`. The fix here could maybe be further improved by writing a simple visitor or (preferably) a pattern-switch when that is a standard feature of the language.
> 
> Fixing the other bug was mostly an accidental side-effect of just using `commentTagsToContent` instead of `CommentHelper.getText`, since the tags now get interpreted instead of ignored.  However, one tweak was necessary.
> The doc comments for serialization info end up in `serialized-form.html` and not in the primary file for the enclosing type.
> This means they should not undergo the standard `redirectRelativeLinks` treatment. Links using `{@link...}` are not affected, but links using explicit `<a href="relative-link">...</a>` are affected. Ideally, we should not be using such relative links in the JDK API documentation, but there are too many to change/fix as part of this work. The fix, for now, is to add a new overload to `commentTagsToContent` that provides the ability to disable the call to `redirectRelativeLinks` when needed ... that is, when generating `serialized-form.html`.
> 
> Initially, the goal was just a cleanup fix with no change to tests. The work has been tested by comparing generated docs before and after this work. There are a number of instances of differences in the generated docs, but all are instances of the bugs described above ... either the `see-list-long` bug, or the change that inline doc comment tags are now interpretedin places where they were previously ignored.  All existing tests continue to work without modification; new tests have been added for the fixes for the bugs that were discovered in the course of this work.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  remove dead code

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9438/files
  - new: https://git.openjdk.org/jdk/pull/9438/files/9e881785..09d03884

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9438&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9438&range=02-03

  Stats: 156 lines in 1 file changed: 0 ins; 156 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9438.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9438/head:pull/9438

PR: https://git.openjdk.org/jdk/pull/9438