Migrate away from `OneofDescriptor.isSynthetic()`

PiperOrigin-RevId: 544416186

java/core/src/main/java/com/google/protobuf/DescriptorMessageInfoFactory.java

@@ -167,7 +167,7 @@ final class DescriptorMessageInfoFactory implements MessageInfoFactory {
               }
             };
       }
-      if (fd.getContainingOneof() != null && !fd.getContainingOneof().isSynthetic()) {
+      if (fd.getRealContainingOneof() != null) {
         // Build a oneof member field for non-synthetic oneofs.
         builder.withField(buildOneofMember(messageType, fd, oneofState, enforceUtf8, enumVerifier));
         continue;

java/core/src/main/java/com/google/protobuf/GeneratedMessageV3.java

@@ -2460,8 +2460,7 @@ public abstract class GeneratedMessageV3 extends AbstractMessage implements Seri
           final Class<? extends Builder<?>> builderClass,
           final String containingOneofCamelCaseName) {
         isOneofField =
-            descriptor.getContainingOneof() != null
+            descriptor.getRealContainingOneof() != null;
-                && !descriptor.getContainingOneof().isSynthetic();
         hasHasMethod = descriptor.hasPresence();
         ReflectionInvoker reflectionInvoker =
             new ReflectionInvoker(