RFR: JDK-8283415: Update java.lang.ref to use sealed classes

[Joe Darcy]

Another refactoring to use sealed classes where possible, this time in the java.lang.ref package.

Copyright dates will be updated, if needed, before a push.

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Commit messages:
 - JDK-8283415: Update java.lang.ref to use sealed classes

Changes: https://git.openjdk.java.net/jdk/pull/7874/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7874&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283415
  Stats: 6 lines in 5 files changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7874.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7874/head:pull/7874

PR: https://git.openjdk.java.net/jdk/pull/7874