Weaken assertion

Turns out there are more situations where we can get junk in constraints, and in at least one of
them (i7969.scala) this happens before errors are reported. So, it's better to just reject junk
types and return false in addConstraint.

Author: odersky

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

@@ -484,9 +484,7 @@ trait ConstraintHandling {
    *  way isSubType is organized.
    */
   protected def addConstraint(param: TypeParamRef, bound: Type, fromBelow: Boolean)(using Context): Boolean =
-    if !bound.isValueTypeOrLambda then
-      assert(ctx.reporter.errorsReported)
-      return false
+    if !bound.isValueTypeOrLambda then return false
 
     /** When comparing lambdas we might get constraints such as
      *  `A <: X0` or `A = List[X0]` where `A` is a constrained parameter