RFR: 8335274: SwitchBootstraps.ResolvedEnumLabels.resolvedEnum should be final

[Aleksey Shipilev]

I was auditing the current uses of `@Stable` before relaxing its barriers ([JDK-8333791](https://bugs.openjdk.org/browse/JDK-8333791)), and this is an easy spot. 

`resolvedEnum` is not `final`. So technically publishing the object via data race can show `resolvedEnum` as `null`, which would break `test()` that does not expect it. Currently not a practical problem since its safety is covered by adjacent `final` fields, but should be more precise.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/19933/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19933&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335274
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19933.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19933/head:pull/19933

PR: https://git.openjdk.org/jdk/pull/19933