RFR: 8357681: Fixed the DigitList::toString method causing incorrect results during debugging

[Shaojin Wen]

When debugging getLong/getDouble/getDecimal of DigitList, the debugger will call the DigitList::toString method. At this time, DigitList::toString will modify tempBuilder, which will cause incorrect results.

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Commit messages:
 - minimal change
 - remove tempBuilder

Changes: https://git.openjdk.org/jdk/pull/25288/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25288&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357681
  Stats: 6 lines in 1 file changed: 0 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25288.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25288/head:pull/25288

PR: https://git.openjdk.org/jdk/pull/25288