Rework cff8b569 to heal the windows build.

 - change newTermName to fix negative length names
   rather than reject them
 - restore the old logic in unspecializedName for names that
   result from AnyRef specialized type parameters.

Why does fix the windows build? I remain none the wiser.
Merge pull request #3013 from retronym/topic/unspec-fix-windows

Rework cff8b569 to heal the windows build.