RFR: 6323374: (coll) Optimize Collections.unmodifiable* and synchronized*

[Ian Graves]

Modify the `unmodifiable*` methods in `java.util.Collections` to be idempotent. That is, when given an immutable collection from `java.util.ImmutableCollections` or `java.util.Collections`, these methods will return the reference instead of creating a new immutable collection that wraps the existing one.

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Commit messages:
 - Changing Collections.unmodifiable* to idempotent behavior.

Changes: https://git.openjdk.java.net/jdk/pull/2596/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2596&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-6323374
  Stats: 281 lines in 2 files changed: 281 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2596.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2596/head:pull/2596

PR: https://git.openjdk.java.net/jdk/pull/2596