nonParamBounds: convert result to Bounds if needed

Be more defensive to handle other issues such as
#4720 (comment).

Code that needs to access the actual entry can call `entry` or `contains`
instead.

Author: Blaisorblade

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

@@ -60,7 +60,7 @@ abstract class Constraint extends Showable {
    *  are not contained in the return bounds.
    *  @pre `param` is not part of the constraint domain.
    */
-  def nonParamBounds(param: TypeParamRef): TypeBounds
+  def nonParamBounds(param: TypeParamRef)(implicit ctx: Context): TypeBounds
 
   /** The lower bound of `param` including all known-to-be-smaller parameters */
   def fullLowerBound(param: TypeParamRef)(implicit ctx: Context): Type

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

@@ -191,8 +191,8 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
   def isLess(param1: TypeParamRef, param2: TypeParamRef): Boolean =
     upper(param1).contains(param2)
 
-  def nonParamBounds(param: TypeParamRef): TypeBounds =
-    entry(param).asInstanceOf[TypeBounds]
+  def nonParamBounds(param: TypeParamRef)(implicit ctx: Context): TypeBounds =
+    entry(param).bounds
 
   def fullLowerBound(param: TypeParamRef)(implicit ctx: Context): Type =
     (nonParamBounds(param).lo /: minLower(param))(_ | _)