SI-9011 Speculative fix for CCE in Scala IDE

Based on the reported stack trace and what I know of Scala IDE,
I've changed `InteractiveNamer#enterExistingSymbol` to be `DocDef`
aware.

I haven't provided a test as this was not minimized from Scala IDE.
Merge pull request #4205 from retronym/ticket/9011

SI-9011 Speculative fix for CCE in Scala IDE