RFR: JDK-8303689: javac -Xlint could/should report on "dangling" doc comments

[Jonathan Gibbons]

Please review the updates to support a proposed new `-Xlint:dangling-doc-comments` option.

The work can be thought of as in 3 parts:

1. An update to the `javac` internal class `DeferredLintHandler` so that it is possible to specify the appropriately configured `Lint` object when it is time to consider whether to generate the diagnostic or not.

2. Updates to the `javac` front end to record "dangling docs comments" found near the beginning of a declaration, and to report them using an instance of `DeferredLintHandler`. This allows the warnings to be enabled or disabled using the standard mechanisms for `-Xlint` and `@SuppressWarnings`.  The procedure for handling dangling doc comments is described in this comment in `JavacParser`.

     *  Dangling documentation comments are handled as follows.
     *  1. {@code Scanner} adds all doc comments to a queue of
     *     recent doc comments. The queue is flushed whenever
     *     it is known that the recent doc comments should be
     *     ignored and should not cause any warnings.
     *  2. The primary documentation comment is the one obtained
     *     from the first token of any declaration.
     *     (using {@code token.getDocComment()}.
     *  3. At the end of the "signature" of the declaration
     *     (that is, before any initialization or body for the
     *     declaration) any other "recent" comments are saved
     *     in a map using the primary comment as a key,
     *     using this method, {@code saveDanglingComments}.
     *  4. When the tree node for the declaration is finally
     *     available, and the primary comment, if any,
     *     is "attached", (in {@link #attach}) any related
     *     dangling comments are also attached to the tree node
     *     by registering them using the {@link #deferredLintHandler}.
     *  5. (Later) Warnings may be genereated for the dangling
     *     comments, subject to the {@code -Xlint} and
     *     {@code @SuppressWarnings}.


3.  Updates to the make files to disable the warnings in modules for which the 
warning is generated.  This is often because of the confusing use of `/**` to
create box or other standout comments.

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

Commit messages:
 - JDK-8303689: javac -Xlint could/should report on "dangling" doc comments

Changes: https://git.openjdk.org/jdk/pull/18527/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18527&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303689
  Stats: 477 lines in 60 files changed: 368 ins; 5 del; 104 mod
  Patch: https://git.openjdk.org/jdk/pull/18527.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18527/head:pull/18527

PR: https://git.openjdk.org/jdk/pull/18527