RFR: 8351000: StringBuilder getChar and putChar robustness [v2]

[Roger Riggs]

> Improve StringBuilder getChar and putChar robustness with racy updates to StringBuilder. 
> Test racing threads with append, insert, and reading characters.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  Test cleanup

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24951/files
  - new: https://git.openjdk.org/jdk/pull/24951/files/0feabe83..e2b842cb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24951&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24951&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24951.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24951/head:pull/24951

PR: https://git.openjdk.org/jdk/pull/24951