RFR: 8293008: Replace uses of StringBuffer with StringBuilder in MergeCollation

[Andrey Turbanov]

Couple of package-private classes in `java.text` package still use `StringBuffer`: `MergeCollation` and `PatternEntry`.
StringBuffer is a legacy synchronized class. StringBuilder is a direct replacement to StringBuffer which generally have better performance.

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Commit messages:
 - [PATCH] Replace uses of StringBuffer with StringBuilder in MergeCollation
 - [PATCH] Replace uses of StringBuffer with StringBuilder in MergeCollation

Changes: https://git.openjdk.org/jdk/pull/10007/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10007&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293008
  Stats: 9 lines in 2 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/10007.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10007/head:pull/10007

PR: https://git.openjdk.org/jdk/pull/10007