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

dotty-staging/dotty,som-snytt/dotty,dotty-staging/dotty,som-snytt/dotty,som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,lampepfl/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty