Avoid merge errors in Scala2 mode

Linearize instead. It would be nice to be able to give a
migration warning here, but unfortunately we do not have
a position at the point the warning would be generated,
and it feels like overkill to do another kind of exception
that gets propagated and caught in typer.

Author: odersky

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

@@ -320,7 +320,7 @@ object Denotations {
                   try info1 & info2
                   catch {
                     case ex: MergeError =>
-                      if (pre.widen.classSymbol.is(Scala2x))
+                      if (pre.widen.classSymbol.is(Scala2x) || ctx.scala2Mode)
                         info1 // follow Scala2 linearization -
                               // compare with way merge is performed in SymDenotation#computeMembersNamed
                       else