RFR: 8335922: Incorrect stable usage of LambdaForm$Name.index

[Chen Liang]

The `@Stable` on the `index` field is incorrect, as stable only avoids inlining `0`. Solution is to use bit flip on the actual index (and rename the field to `flippedIndex`), so we use -1 to -256 (mapping to 0 to 255) and 0 the default value is used as an unset indicator.

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Commit messages:
 - 8335922: Incorrect stable usage of LambdaForm$Name.index

Changes: https://git.openjdk.org/jdk/pull/20178/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20178&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335922
  Stats: 21 lines in 2 files changed: 0 ins; 0 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/20178.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20178/head:pull/20178

PR: https://git.openjdk.org/jdk/pull/20178