Make CLASS default class kind in binary metadata

Don't write the kind to the class file if it's CLASS to save some bytes

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

@@ -239,13 +239,20 @@ public class ImplementationBodyCodegen extends ClassBodyCodegen {
             kind = KotlinClass.Kind.ANONYMOUS_OBJECT;
         }
         else if (isTopLevelOrInnerClass(descriptor)) {
-            kind = KotlinClass.Kind.CLASS;
+            // Default value is Kind.CLASS
+            kind = null;
         }
         else {
             // LOCAL_CLASS is also written to inner classes of local classes
             kind = KotlinClass.Kind.LOCAL_CLASS;
         }
 
+        // Temporarily write class kind anyway because old compiler may not expect its absence
+        // TODO: remove after M13
+        if (kind == null) {
+            kind = KotlinClass.Kind.CLASS;
+        }
+
         DescriptorSerializer serializer =
                 DescriptorSerializer.create(descriptor, new JvmSerializerExtension(v.getSerializationBindings(), typeMapper));
 
@@ -257,11 +264,14 @@ public class ImplementationBodyCodegen extends ClassBodyCodegen {
 
         AnnotationVisitor av = v.getVisitor().visitAnnotation(asmDescByFqNameWithoutInnerClasses(JvmAnnotationNames.KOTLIN_CLASS), true);
         av.visit(JvmAnnotationNames.ABI_VERSION_FIELD_NAME, JvmAbi.VERSION);
-        av.visitEnum(
-                JvmAnnotationNames.KIND_FIELD_NAME,
-                Type.getObjectType(KotlinClass.KIND_INTERNAL_NAME).getDescriptor(),
-                kind.toString()
-        );
+        //noinspection ConstantConditions
+        if (kind != null) {
+            av.visitEnum(
+                    JvmAnnotationNames.KIND_FIELD_NAME,
+                    Type.getObjectType(KotlinClass.KIND_INTERNAL_NAME).getDescriptor(),
+                    kind.toString()
+            );
+        }
         AnnotationVisitor array = av.visitArray(JvmAnnotationNames.DATA_FIELD_NAME);
         for (String string : BitEncoding.encodeBytes(SerializationUtil.serializeClassData(data))) {
             array.visit(null, string);