TypedTreeCopier#Select: don't use unstable prefixes

This manifested itself as a pickling difference in tasty_tools

Note that there are probably more issues in this method, in particular
the old type is reused when `qualifier.tpe eq tree.qualifier.tpe` even
if the `name` is different. But I'm only trying to get the tests to pass
for now.

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -464,7 +464,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
         case tree: Select if qualifier.tpe eq tree.qualifier.tpe =>
           tree1.withTypeUnchecked(tree.tpe)
         case _ => tree.tpe match {
-          case tpe: NamedType => tree1.withType(tpe.derivedSelect(qualifier.tpe))
+          case tpe: NamedType => tree1.withType(tpe.derivedSelect(qualifier.tpe.widenIfUnstable))
           case _ => tree1.withTypeUnchecked(tree.tpe)
         }
       }