Tweak refineUsingParent drop conditions

In tests/pos/i21790.scala, O.A has no class symbol, because it's an
inaccessible private class of S.  But in tests/warn/i21860.scala,
Shape.Triangle doesn't have a "classSymbol" because it has multiple -
Shape and Corners.  So use .classSymbols.isEmpty instead of
!.classSymbol.exists

[Cherry-picked 21e5f3c]

Author: dwijnand

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

@@ -944,7 +944,7 @@ object TypeOps:
         protoTp1 <:< tp
       maximizeType(protoTp1, NoSpan)
       val inst = wildApprox(protoTp1)
-      if !inst.classSymbol.exists then
+      if inst.classSymbols.isEmpty then
         // E.g. i21790, can't instantiate S#CA as a subtype of O.A, because O.CA isn't accessible
         NoType
       else inst