RFR: JDK-8310835: Address gaps in -Xlint:serial checks [v4]

[Jonathan Gibbons]

On Sat, 15 Jul 2023 02:05:24 GMT, Joe Darcy <darcy at openjdk.org> wrote:

>> 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:
> 
>   Refactor Externalizable check.

Marked as reviewed by jjg (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/14639#pullrequestreview-1535589080