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

[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.

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Commit messages:
 - JDK-8254052: improve type specificity of TagletWriter and friends

Changes: https://git.openjdk.java.net/jdk/pull/517/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=517&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254052
  Stats: 127 lines in 17 files changed: 51 ins; 8 del; 68 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