RFR: 8303755: Clean up around JavacElements.getAllMembers [v3]

[Pavel Rappo]

> Please review this cleanup which consists of commentary changes and code simplifications.

Pavel Rappo has updated the pull request incrementally with one additional commit since the last revision:

  Respond to feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12904/files
  - new: https://git.openjdk.org/jdk/pull/12904/files/6515bf76..140afc8a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12904&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12904&range=01-02

  Stats: 6 lines in 2 files changed: 1 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12904.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12904/head:pull/12904

PR: https://git.openjdk.org/jdk/pull/12904