[FE 1.0] Refactoring: replace defensive NonPrivateCallableAdded with Unknown

Motivation: I'm going to drop NonPrivateCallableAdded (KT-62655) in the
next commits. But I don't want to change the existing logic when I drop
it. That's why I have to introduce the "Unknown" case

core/compiler.common/src/org/jetbrains/kotlin/resolve/multiplatform/ExpectActualMemberDiff.kt

@@ -53,6 +53,10 @@ data class ExpectActualMemberDiff<out M, out C>(val kind: Kind, val actualMember
             "{0}: the type parameter names of this member must be the same in the expect class and the actual class. " +
                     "This error happens because the expect class ''{1}'' is non-final"
         ),
+        Unknown(
+            "{0}: normally, this error should never happen. Please report to https://kotl.in/issue. " +
+                    "This error happens because the expect class ''{1}'' is non-final"
+        )
     }
 }