RFR: 8341664: ReferenceClassDescImpl cache internalName

[Shaojin Wen]

ReferenceClassDescImpl caches internalName to avoid creating a new String object for each call, which can also simplify the implementation of classfile.

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Commit messages:
 - suggestion from @liach
 - stable
 - ReferenceClassDescImpl cache internalName

Changes: https://git.openjdk.org/jdk/pull/21373/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21373&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341664
  Stats: 97 lines in 5 files changed: 14 ins; 74 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/21373.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21373/head:pull/21373

PR: https://git.openjdk.org/jdk/pull/21373