Exclude nested classes of private classes from ABI classes

compiler/serialization/src/org/jetbrains/kotlin/serialization/DescriptorSerializer.kt

@@ -119,6 +119,10 @@ class DescriptorSerializer private constructor(
                 typeAliasProto(descriptor)?.let { builder.addTypeAlias(it) }
             }
             else {
+                if (descriptor is ClassDescriptor && !extension.shouldSerializeNestedClass(descriptor)) {
+                    continue
+                }
+
                 val name = getSimpleNameIndex(descriptor.name)
                 if (isEnumEntry(descriptor)) {
                     builder.addEnumEntry(enumEntryProto(descriptor as ClassDescriptor))

compiler/serialization/src/org/jetbrains/kotlin/serialization/SerializerExtension.kt

@@ -25,6 +25,7 @@ abstract class SerializerExtension {
     open fun shouldSerializeFunction(descriptor: FunctionDescriptor): Boolean = true
     open fun shouldSerializeProperty(descriptor: PropertyDescriptor): Boolean = true
     open fun shouldSerializeTypeAlias(descriptor: TypeAliasDescriptor): Boolean = true
+    open fun shouldSerializeNestedClass(descriptor: ClassDescriptor): Boolean = true
 
     interface ClassMembersProducer {
         fun getCallableMembers(classDescriptor: ClassDescriptor): Collection<CallableMemberDescriptor>