RFR: 8304146: Refactor VisibleMemberTable (LocalMemberTable)

[Pavel Rappo]

Please review a change to clean up and simplify LocalMemberTable; a container to cache, classify, and provide efficient lookup for the return value of `TypeElement.getEnclosedElements()`.

While the change primarily targets internals of LocalMemberTable, it also affects its clients: in particular, code that handles JavaFX documentation. That code does not seem to be tested well (I filed a bug for that: JDK-8304170). To make sure I haven't broken anything, aside from usual testing, I also cloned [OpenJFX](https://github.com/openjdk/jfx) and built its documentation with javadoc before and after the change. Documentation bundles were identical.

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Commit messages:
 - Improve LocalMemberTable

Changes: https://git.openjdk.org/jdk/pull/13044/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13044&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304146
  Stats: 96 lines in 1 file changed: 13 ins; 47 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/13044.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13044/head:pull/13044

PR: https://git.openjdk.org/jdk/pull/13044