Avoid pickling error

Making `newConstraint` private caused its default getters to be `private[this]` before pickling
but `private` after pickling. Need to follow up on why that is.

Author: odersky

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

@@ -138,7 +138,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
   type This = OrderingConstraint
 
   /** A new constraint with given maps and given set of hard typevars */
-  private def newConstraint(
+  def newConstraint( // !!! Dotty problem: Making newConstraint `private` causes -Ytest-pickler failure.
     boundsMap: ParamBounds = this.boundsMap,
     lowerMap: ParamOrdering = this.lowerMap,
     upperMap: ParamOrdering = this.upperMap,