RFR: JDK-8322512 StringBuffer.repeat does not work correctly after toString() was called

[Jim Laskey]

The new repeat methods were not clearing the toStringCache.

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Commit messages:
 - Clear toStringCache when calling StringBuffer::repeat

Changes: https://git.openjdk.org/jdk/pull/17172/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17172&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/jdk/pull/17172.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17172/head:pull/17172

PR: https://git.openjdk.org/jdk/pull/17172