[jdk21u-dev] RFR: 8322512: StringBuffer.repeat does not work correctly after toString() was called

[Aleksey Shipilev]

Clean backport to fix JDK 21 feature misbehavior.

Additional testing:
 - [x] New regression test fails without the fix, passes with it
 - [ ] GHA (includes some Repeat tests)

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Commit messages:
 - Backport df22fb322e6c4c9931a770bd0abf4c43b83c4e4a

Changes: https://git.openjdk.org/jdk21u-dev/pull/143/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=143&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322512
  Stats: 16 lines in 2 files changed: 15 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/143.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/143/head:pull/143

PR: https://git.openjdk.org/jdk21u-dev/pull/143