RFR: JDK-8254052: improve type specificity of TagletWriter and friends [v3]

[Jonathan Gibbons]

> The types of `DocTree` parameters in some methods in `TagletWriter` can be changed from `DocTree` to more specific
> subtypes. Doing so makes the code less prone to incorrect calls, and removes the need for downcasting the argument in
> the method bodies.  This means the implementation of those methods can often be simplified.
> 
> In addition, the call sites need to be improved, and with the help of some improvements to methods in `Utils`, the
> methods that access different kinds of trees can return the lists using a more specific subtype as well.
> Finally, a quick analysis of some of the `Utils` methods to access different kinds of trees shows that there are some
> more convenient methods that can be used instead.

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

  review feedback: improve type specificity of use of utils.get*Trees

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/517/files
  - new: https://git.openjdk.java.net/jdk/pull/517/files/5e9ee9f4..b441ad42

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=517&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=517&range=01-02

  Stats: 34 lines in 9 files changed: 5 ins; 10 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/517.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/517/head:pull/517

PR: https://git.openjdk.java.net/jdk/pull/517