RFR: JDK-8310835: Address gaps in -Xlint:serial checks [v3]

[Joe Darcy]

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

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Add support for externalizable enum checks.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14639/files
  - new: https://git.openjdk.org/jdk/pull/14639/files/6196f9b8..17b6dfd7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14639&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14639&range=01-02

  Stats: 78 lines in 6 files changed: 75 ins; 0 del; 3 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