Fixes "slow git pull while type hierarchy view is open"
https://github.com/eclipse-jdt/eclipse.jdt.core/issues/1779

Use LinkedHashSet / HashSet to avoid linear search on contains().
Javadoc of ITypeHierarchy.getAllSubtypes(IType) states: return "in no
particular order"

tested by TypeHierarchyTests, TypeHierarchySerializationTests