[jdk21u-dev] RFR: 8351000: StringBuilder getChar and putChar robustness

[Goetz Lindenmaier]

Clean except of Copyright, but removing import breaks build as
    8351443: Improve robustness of StringBuilder
is not in 21.

Wouldn't this make sense anyways if this one is backported? Adresses
a similar issue, but some more intrusive.

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Commit messages:
 - fix build
 - backport 4c695fa8a459adcdb8cdfe9e90783007c65fb90e

Changes: https://git.openjdk.org/jdk21u-dev/pull/2516/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2516&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351000
  Stats: 9 lines in 1 file changed: 5 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2516.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2516/head:pull/2516

PR: https://git.openjdk.org/jdk21u-dev/pull/2516