RFR: JDK-8310835: Address gaps in -Xlint:serial checks

[Joe Darcy]

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

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Commit messages:
 - Merge branch 'master' into JDK-8310835
 - Refactor implementation and add test.
 - Remove unused method.
 - Merge branch 'master' into JDK-8310835
 - Fix method call.
 - Merge branch 'master' into JDK-8310835
 - Checkpoint.
 - Update tests.
 - JDK-8310835: Address gaps in -Xlint:serial checks

Changes: https://git.openjdk.org/jdk/pull/14639/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14639&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310835
  Stats: 164 lines in 6 files changed: 162 ins; 1 del; 1 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