Turn off F-bound wildcard skolemizatiuon under Scala-2 mode

With skolemization we fail compiling the standard lib with:
```
[E057] Type Mismatch Error: /Users/odersky/workspace/dotty/tests/scala2-library/src/library/scala/collection/generic/ParMapFactory.scala:28:76
28 |abstract class ParMapFactory[CC[X, Y] <: ParMap[X, Y] with ParMapLike[X, Y, CC[X, Y], _]]
   |                                                                            ^
   |Type argument CC[X, Y] does not conform to upper bound scala.collection.parallel.ParMapLike[X, Y, LazyRef(CC[X, Y]),
   |  collection.parallel.ParMapLike[X, Y, CC[X, Y], _]#Sequential
   |] & scala.collection.parallel.ParMap[X, Y]
--
```
I have no idea whether this bound is correct or not, and am not inclined to find out. But we clearly have to support it
anyway under Scala-2 mode.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -271,7 +271,7 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
      *  type parameter corresponding to the wildcard.
      */
     def skolemizeWildcardArgs(tps: List[Type], app: Type) = app match {
-      case AppliedType(tycon, args) if tycon.typeSymbol.isClass =>
+      case AppliedType(tycon, args) if tycon.typeSymbol.isClass && !scala2Mode =>
         tps.zipWithConserve(tycon.typeSymbol.typeParams) {
           (tp, tparam) => tp match {
             case _: TypeBounds => app.select(tparam)
@@ -321,6 +321,8 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
             def massage(tp: Type): Type = tp match {
               case tp @ AppliedType(tycon, args) =>
                 tp.derivedAppliedType(tycon, skolemizeWildcardArgs(args, tp))
+              case tp: AndOrType =>
+                tp.derivedAndOrType(massage(tp.tp1), massage(tp.tp2))
               case _ => tp
             }
             def narrowBound(bound: Type, fromBelow: Boolean): Unit = {