Add an allowlist for implementation reserved fields for the C++ implementation.

This also includes `protoc`.
The allowlist is currently empty.

PiperOrigin-RevId: 493635020

src/google/protobuf/descriptor.cc

@@ -5487,6 +5487,12 @@ void DescriptorBuilder::BuildMessage(const DescriptorProto& proto,
   }
 }
 
+namespace {
+bool IsAllowedReservedField(const FieldDescriptorProto& field) {
+  return false;
+}
+}  // namespace
+
 void DescriptorBuilder::BuildFieldOrExtension(const FieldDescriptorProto& proto,
                                               Descriptor* parent,
                                               FieldDescriptor* result,
@@ -5713,7 +5719,8 @@ void DescriptorBuilder::BuildFieldOrExtension(const FieldDescriptorProto& proto,
              absl::Substitute("Field numbers cannot be greater than $0.",
                               FieldDescriptor::kMaxNumber));
   } else if (result->number() >= FieldDescriptor::kFirstReservedNumber &&
-             result->number() <= FieldDescriptor::kLastReservedNumber) {
+             result->number() <= FieldDescriptor::kLastReservedNumber &&
+             !IsAllowedReservedField(proto)) {
     message_hints_[parent].RequestHintOnFieldNumbers(
         proto, DescriptorPool::ErrorCollector::NUMBER);
     AddError(result->full_name(), proto, DescriptorPool::ErrorCollector::NUMBER,

src/google/protobuf/descriptor_unittest.cc

@@ -4601,6 +4601,7 @@ TEST_F(ValidationErrorTest, ReservedFieldNumber) {
       "foo.proto: Foo: NUMBER: Suggested field numbers for Foo: 1, 2\n");
 }
 
+
 TEST_F(ValidationErrorTest, ExtensionMissingExtendee) {
   BuildFileWithErrors(
       "name: \"foo.proto\" "