RFR: 8351443: Improve robustness of StringBuilder [v3]

[Roger Riggs]

> Refactor AbstractStringBuilder to maintain consistency among count, coder, and value buffers while the buffer capacity is being expanded and/or inflated from Latin1 to UTF16 representations. 
> The refactoring pattern is to read and write AbstractStringBuilder fields once using locals for all intermediate values. 
> Support methods are static, designed to pass all values as arguments and return a value.
> 
> The value byte array is reallocated under 3 conditions:
> - Increasing the capacity with the same encoder
> - Increasing the capacity and inflation to change the coder from LATIN1 to UTF16
> - Inflation with the same capacity
> 
> Added StressSBTest to exercise public instance methods of StringBuilder.

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

  Optimize StringUTF16.putCharsAt a bit.
  Fixup hotspot Helper of putCharsAt to remove return value to match StringUTF16.putCharsAt.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24967/files
  - new: https://git.openjdk.org/jdk/pull/24967/files/f5003244..d2261cd6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24967&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24967&range=01-02

  Stats: 15 lines in 2 files changed: 0 ins; 2 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/24967.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24967/head:pull/24967

PR: https://git.openjdk.org/jdk/pull/24967