Use some sealed class

I'm playing with the code of JDT-LS and am starting to see some patterns where newer Java would serve JDT-Core and its consumer well.
One of them is usage of Sealed class for classes that are @noextends . This issue aims at collecting some possible candidates.

* org.eclipse.jdt.core.dom.Name
* org.eclipse.jdt.core.dom.IBinding and family

Note that it's no surprise that the parser is the main area when we see this pattern. Same analysis led to same conclusions for eg LemMinX. Looks like parsing is in general a lot around pattern matching and resticted types of objects; so sealed type, instanceof patterns, switch expressions on type... are very likely to serve well.