RFR: JDK-8312415: Expand -Xlint:serial checks to enum constants with specialized class bodies

[Joe Darcy]

Thanks to @lahodaj for providing the code to bridge into the class used for a enum constant with a specialized body.

This changes expands serialization warnings to the contents of any such class bodies.

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Commit messages:
 - Appease jcheck.
 - JDK-8312415: Expand -Xlint:serial checks to enum constants with specialized class bodies

Changes: https://git.openjdk.org/jdk/pull/15004/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15004&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312415
  Stats: 145 lines in 3 files changed: 145 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15004.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15004/head:pull/15004

PR: https://git.openjdk.org/jdk/pull/15004