RFR: 8132995: Matcher$ImmutableMatchResult should be optimized to reduce space usage

[Raffaello Giulietti]

When appropriate and useful, copies only the relevant portion of the `CharSequence` to the match result.

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Commit messages:
 - 8132995: Matcher should be optimized to reduce space usage

Changes: https://git.openjdk.org/jdk/pull/13231/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13231&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8132995
  Stats: 111 lines in 2 files changed: 97 ins; 2 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/13231.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13231/head:pull/13231

PR: https://git.openjdk.org/jdk/pull/13231