RFR: JDK-8310835: Address gaps in -Xlint:serial checks [v5]

[Joe Darcy]

> Improve serial lint messages for externalizable records (and potentially other situations).

Joe Darcy has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 14 commits:

 - Respond to review feedback; make messages for enum externalizable methods more precise.
 - Merge branch 'master' into JDK-8310835
 - Refactor Externalizable check.
 - Add support for externalizable enum checks.
 - Fix typo found in code review.
 - Merge branch 'master' into JDK-8310835
 - Refactor implementation and add test.
 - Remove unused method.
 - Merge branch 'master' into JDK-8310835
 - Fix method call.
 - ... and 4 more: https://git.openjdk.org/jdk/compare/b5b6f4e7...2c90546a

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Changes: https://git.openjdk.org/jdk/pull/14639/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14639&range=04
  Stats: 322 lines in 10 files changed: 306 ins; 9 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/14639.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14639/head:pull/14639

PR: https://git.openjdk.org/jdk/pull/14639