RFR: 8371431: Warning message when turning off CompactStrings [v2]

[Shaojin Wen]

> A warning message should be given before removing the CompactStrings off option.

Shaojin Wen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge branch 'master' into compact_str_warn_2510
 - Merge branch 'master' into compact_str_warn_2510
 - from @liach
 - add warnings

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Changes: https://git.openjdk.org/jdk/pull/27995/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27995&range=01
  Stats: 12 lines in 2 files changed: 0 ins; 9 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/27995.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27995/head:pull/27995

PR: https://git.openjdk.org/jdk/pull/27995