kotlinx.serialization: Support @Serializable on sealed interfaces.

Interfaces (regular and sealed) are by default polymorphic. To benefit
from sealing (i.e. knowledge of all inheritors in compile-time), @Serializable
annotation may be added on sealed interface, generating the same serializer
that can be used for sealed classes.

Synthetic nested classes are not generated in DEFAULT_IMPLS mode because
it causes problems when adding a synthetic companion to an interface.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/1576

compiler/backend/src/org/jetbrains/kotlin/codegen/ClassBodyCodegen.java

@@ -105,12 +105,14 @@ public abstract class ClassBodyCodegen extends MemberCodegen<KtPureClassOrObject
         }
 
         // Generate synthetic nested classes
-        Collection<DeclarationDescriptor> classifiers = descriptor
-                .getUnsubstitutedMemberScope()
-                .getContributedDescriptors(DescriptorKindFilter.CLASSIFIERS, MemberScope.Companion.getALL_NAME_FILTER());
-        for (DeclarationDescriptor memberDescriptor : classifiers) {
-            if (memberDescriptor instanceof SyntheticClassOrObjectDescriptor) {
-                genSyntheticClassOrObject((SyntheticClassOrObjectDescriptor) memberDescriptor);
+        if (kind != OwnerKind.DEFAULT_IMPLS) {
+            Collection<DeclarationDescriptor> classifiers = descriptor
+                    .getUnsubstitutedMemberScope()
+                    .getContributedDescriptors(DescriptorKindFilter.CLASSIFIERS, MemberScope.Companion.getALL_NAME_FILTER());
+            for (DeclarationDescriptor memberDescriptor : classifiers) {
+                if (memberDescriptor instanceof SyntheticClassOrObjectDescriptor) {
+                    genSyntheticClassOrObject((SyntheticClassOrObjectDescriptor) memberDescriptor);
+                }
             }
         }