protoc-c: Use FileDescriptorLegacy to obtain proto syntax version on protobuf >= 23.0

Use the newer "legacy" way of determining whether a file descriptor is
using proto2 or proto3 syntax on protobuf >= 23.0.

Based on
https://github.com/protobuf-c/protobuf-c/pull/556/commits/66574f3fd85a205eb7c90b790477d5415364209e
but continues to support older versions of protobuf.

Unfortunately, since this is a "deprecated", "legacy" API it'll probably
disappear in about five seconds.

protoc-c/c_file.cc

@@ -119,7 +119,11 @@ void FileGenerator::GenerateHeader(io::Printer* printer) {
 
   int min_header_version = 1000000;
 #if defined(HAVE_PROTO3)
+# if GOOGLE_PROTOBUF_VERSION >= 4023000
+  if (FileDescriptorLegacy(file_).syntax() == FileDescriptorLegacy::SYNTAX_PROTO3) {
+# else
   if (file_->syntax() == FileDescriptor::SYNTAX_PROTO3) {
+#endif
     min_header_version = 1003000;
   }
 #endif

protoc-c/c_helpers.h

@@ -70,6 +70,10 @@
 #include <protobuf-c/protobuf-c.pb.h>
 #include <google/protobuf/io/printer.h>
 
+#if GOOGLE_PROTOBUF_VERSION >= 4023000
+# include <google/protobuf/descriptor_legacy.h>
+#endif
+
 namespace google {
 namespace protobuf {
 namespace compiler {
@@ -172,7 +176,11 @@ int compare_name_indices_by_name(const void*, const void*);
 // This wrapper is needed to be able to compile against protobuf2.
 inline int FieldSyntax(const FieldDescriptor* field) {
 #ifdef HAVE_PROTO3
+# if GOOGLE_PROTOBUF_VERSION >= 4023000
+  return FileDescriptorLegacy(field->file()).syntax() == FileDescriptorLegacy::SYNTAX_PROTO3 ? 3 : 2;
+# else
   return field->file()->syntax() == FileDescriptor::SYNTAX_PROTO3 ? 3 : 2;
+# endif
 #else
   return 2;
 #endif