RFR: 8340024: In ClassReader, extract a constant for the superclass supertype_index

[Liam Miller-Cushon]

Hi, please consider this cleanup to extract a literal to a documented constant in `ClassReader`, see [JDK-8340024](https://bugs.openjdk.org/browse/JDK-8340024).

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Commit messages:
 - 8340024: In ClassReader, extract a constant for the superclass supertype_index

Changes: https://git.openjdk.org/jdk/pull/20970/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20970&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340024
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20970.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20970/head:pull/20970

PR: https://git.openjdk.org/jdk/pull/20970