Avoid stale symbol errors for Scala-2 compiled symbols

Wehn compiling dotty repeatedly using dotc.Bench, we got a stale symbol
error involving an outer field of a Scala class. These can always be assumed
to be current, as we would not think to recompile Scala2 classes mixed
with dotty classes in the same session (we'd need two concurrent
compilers for that).

Author: odersky

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -56,6 +56,7 @@ trait SymDenotations { this: Context =>
     stillValid(owner) && (
       !owner.isClass
       || owner.isRefinementClass
+      || owner.is(Scala2x)
       || (owner.unforcedDecls.lookupAll(denot.name) contains denot.symbol)
       || denot.isSelfSym)
   } catch {