RFR: 8351443: Improve robustness of StringBuilder [v4]

[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:

  Applied suggestion to use StringUTF16.compress in append() for copying char[].
  Added similar improvement to putCharsAt.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24967/files
  - new: https://git.openjdk.org/jdk/pull/24967/files/d2261cd6..ff3d841f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24967&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24967&range=02-03

  Stats: 10 lines in 1 file changed: 8 ins; 0 del; 2 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