Backport #2289's TermNames.unexpandedName as TermNames.originalName

Because this implementation is more clear than 2.10.x's and it will
simplify a further commit to fix SI-6715.
SI-6715 Shouldn't return "" from TermNames.originalName
Merge pull request #2281 from kzys/jira-6715

SI-6715 Scaladoc: Use an encoded name if a decoded name is ""