Use public methods to fetch oneofs in generated code.

When building into frameworks, the generated code doesn't always have direct
access to the proto internals.  Instead of opening up the access, just use the
public method to fetch the correct oneof.

Fixes https://github.com/google/protobuf/issues/1789

objectivec/google/protobuf/Struct.pbobjc.m

@@ -237,7 +237,7 @@ void SetGPBValue_NullValue_RawValue(GPBValue *message, int32_t value) {
 
 void GPBValue_ClearKindOneOfCase(GPBValue *message) {
   GPBDescriptor *descriptor = [message descriptor];
-  GPBOneofDescriptor *oneof = descriptor->oneofs_[0];
+  GPBOneofDescriptor *oneof = [descriptor.oneofs objectAtIndex:0];
   GPBMaybeClearOneof(message, oneof, -1, 0);
 }
 #pragma mark - GPBListValue

src/google/protobuf/compiler/objectivec/objectivec_oneof.cc

@@ -119,7 +119,7 @@ void OneofGenerator::GenerateClearFunctionImplementation(io::Printer* printer) {
       variables_,
       "void $owning_message_class$_Clear$capitalized_name$OneOfCase($owning_message_class$ *message) {\n"
       "  GPBDescriptor *descriptor = [message descriptor];\n"
-      "  GPBOneofDescriptor *oneof = descriptor->oneofs_[$raw_index$];\n"
+      "  GPBOneofDescriptor *oneof = [descriptor.oneofs objectAtIndex:$raw_index$];\n"
       "  GPBMaybeClearOneof(message, oneof, $index$, 0);\n"
       "}\n");
 }