RFR: 8224243: Make AccessibleObject a sealed class [v2]

[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

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Update in response to review feedback.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4133/files
  - new: https://git.openjdk.java.net/jdk/pull/4133/files/f605cfa8..be517cda

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4133&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4133&range=00-01

  Stats: 21 lines in 3 files changed: 15 ins; 0 del; 6 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