RFR: 8224243: Make AccessibleObject a sealed class

[Joe Darcy]

Conceptually, AccessbileObject is a sealed class with a protected constructor stating

    Constructor: only used by the Java Virtual Machine.

With the language now supporting sealed classes, the AccessbileObject should be marked as sealed.

Executable and Field are the subclasses of AccessbileObject in the JDK; as Executable has subclasses, it is marked as non-sealed.

Please also review the corresponding CSR:

https://bugs.openjdk.java.net/browse/JDK-8224243

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Commit messages:
 - 8224243: Make AccessibleObject a sealed class

Changes: https://git.openjdk.java.net/jdk/pull/4133/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4133&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8224243
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4133.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4133/head:pull/4133

PR: https://git.openjdk.java.net/jdk/pull/4133