Revert "Drop inaccessible subclasses from refineUsingParent" (scala#21895)

Reverts scala#21799

The following changes were merged at the ~same time as adding the
`tests/warn/i21860.scala` test, which are breaking that test. We revert
it for now to unblock merging to `main`.

Author: hamzaremmal

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

@@ -292,7 +292,7 @@ object Decorators {
       case _ => String.valueOf(x).nn
 
     /** Returns the simple class name of `x`. */
-    def className: String = if x == null then "<null>" else x.getClass.getSimpleName.nn
+    def className: String = x.getClass.getSimpleName.nn
 
   extension [T](x: T)
     def assertingErrorsReported(using Context): T = {

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

@@ -930,11 +930,7 @@ object TypeOps:
       for tp <- mixins.reverseIterator do
         protoTp1 <:< tp
       maximizeType(protoTp1, NoSpan)
-      val inst = wildApprox(protoTp1)
-      if !inst.classSymbol.exists then
-        // E.g. i21790, can't instantiate S#CA as a subtype of O.A, because O.CA isn't accessible
-        NoType
-      else inst
+      wildApprox(protoTp1)
     }
 
     if (protoTp1 <:< tp2) instantiate()