RFR: 8307652: sealed class hierarchy graph doesn't distinguish non-sealed classes

[Chen Liang]

`@sealedGraph` had a mechanism to render non-sealed classes differently, but it's useless because the graph nodes are not bordered. This patch converts the non-sealed classes to be rendered in italics instead.

An example of `ConstantDesc`, which has a sealed hierarchy except `DynamicConstantDesc`:
JDK 20:
![image](https://user-images.githubusercontent.com/7806504/236991678-e30c181a-cb1f-407a-b3e0-f648fe2df788.png)

This patch:
![image](https://user-images.githubusercontent.com/7806504/236991592-affcb128-9721-45cf-860c-6292ee6a8bb6.png)

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Commit messages:
 - 8307652: sealed class hierarchy graph doesn't distinguish non-sealed classes

Changes: https://git.openjdk.org/jdk/pull/13877/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13877&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307652
  Stats: 30 lines in 1 file changed: 22 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13877.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13877/head:pull/13877

PR: https://git.openjdk.org/jdk/pull/13877