RFR: JDK-8268420: new Reporter method to report a diagnostic within a DocTree node

[Jonathan Gibbons]

Please review an update to add a new method in Reporter to report a diagnostic within a DocTree node for those DocTree nodes that wrap a string.

This is the last of the current round of updates to improve the diagnostics that can be generated by javadoc.

The general fix, in JavadocLog and Reporter, is pretty simple, given all the improvements in recent related changes.

There are some cosmetic cleanups that were made while exploring the current solution.

The test is "reasonably thorough" and uses a custom taglet to generate diagnostics for selected nodes in doc comment trees. The test then "algorithmically validates" (i.e. no golden files or text blocks) the diagnostics that are either passed to a DiagnosticListener or written to the console stream.

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

Commit messages:
 - JDK-8268420

Changes: https://git.openjdk.java.net/jdk/pull/4489/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4489&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268420
  Stats: 542 lines in 8 files changed: 528 ins; 5 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4489.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4489/head:pull/4489

PR: https://git.openjdk.java.net/jdk/pull/4489