RFR: 8303755: Clean up around JavacElements.getAllMembers [v2]

[Pavel Rappo]

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

Pavel Rappo has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:

 - Merge branch 'master' into 8303755
   
   # Conflicts:
   #	src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java
 - Improve CompoundIterator
   
   - it could be improved even further, but kept as is to retain its
     laziness
   - `remove` is deleted as it now is a default Iterator method
 - Improve Iterators.createFilterIterator
   
   Assert that the mechanics relies on iterator being filtered never
   to return null.
 - Fix typos and improve consistency
 - Avoid unnecessary recomputation
 - Fix typos and {@inheritDoc} that stands out

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Changes: https://git.openjdk.org/jdk/pull/12904/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12904&range=01
  Stats: 49 lines in 6 files changed: 9 ins; 20 del; 20 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