Adding a second item-template file (Form, Class, etc.) to an SDK-style project can freeze the IDE

[fforay]

Adding two or more files based on any X# item template that lists a block (Form, Class, UserControl, WPF items, etc.) to the same SDK-style (.NET Core) project can freeze Visual Studio permanently.

To Reproduce

1. Create any SDK-style (.NET Core) X# WinForms/console project.
2. Add New Item -> a second file of any template that lists
   (e.g. a second Class, or a second Form).
3. Add a third such item, or do anything else that needs to enumerate the
   References/Dependencies node -- IDE freezes.

Diagnosed via live debugger attach: a reference node's own nextSibling
field pointed back to itself.

Fix:
guard HierarchyNode.AddChild against ever re-linking a node that's already a member of the target children list.