RFR: 8297150: Add a @sealedGraph tag to Reference

[Per Minborg]

This PR proposes to opt in for graphic rendering of the sealed hierarchy of the 
`Reference` class.

Rendering capability was added via https://bugs.openjdk.org/browse/JDK-8295653

Here is how it would look like:

<img width="449" alt="Reference_SH" 
src="https://user-images.githubusercontent.com/7457876/202243138-7f2a799a-dc67-4c4e-9322-c18bf05d73e7.png";>

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

Commit messages:
 - Add a @sealedGraph tag to Reference

Changes: https://git.openjdk.org/jdk/pull/11191/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11191&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297150
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11191.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11191/head:pull/11191

PR: https://git.openjdk.org/jdk/pull/11191