RFR: JDK-8261079: Fix support for @hidden in classes and interfaces

[Hannes Wallnöfer]

This change improves support for the `@hidden` tag to suppress documentation for specific elements, especially in the context of classes, interfaces and inheritance/method overriding.

The important changes are in `Utils` and in `VisibleMemberTable`. (There is also a number of non-related small code cleanup changes, sorry about that, I know it makes review a bit harder but I couldn't resist.)

In `Utils` the most important change are:

 - Consider types as "undocumented enclosures" if they are marked with a `@hidden` tag
 - Check for `@hidden` tags even in non-included elements as they may be included via undocumented enclosures
 - To counter the above change, only run doclint on elements that are either included or contained in an included type, as we don't want to report warnings or errors for non-included code. 
 
In `VisibleMemberTable` there is a subtle change to not consider an overriding method as a "simple override" if the overridden method is hidden or otherwise invisible but in turn is a non-simple override of a method further up the inheritance chain. This resulted in methods which should have been documented as declared locally to be documented as declared in supertypes. I also did a bit of renaming in `VisibleMemberTable` to make the purpose of things a bit clearer.

Other than that, most of the changes consist of added calls to `utils.hasHiddenTag(Element)`, usually with the purpose of not generating links to things that are not there.

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Commit messages:
 - JDK-8261079: Finish tests and small fixes
 - JDK-8261079: Fix support for @hidden in classes and interfaces

Changes: https://git.openjdk.java.net/jdk/pull/2406/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2406&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261079
  Stats: 430 lines in 16 files changed: 303 ins; 35 del; 92 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2406.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2406/head:pull/2406

PR: https://git.openjdk.java.net/jdk/pull/2406