Reimplement FieldSyntax() to maximize compatibility across protobuf versions

Recent versions of Google protobuf have broken the interfaces for
determining the syntax version of a .proto file. The current protobuf-c
1.5.0 release does not compile with Google protobuf 26.0 due to the most
recentage breakage. There is a possible workaround involving the Google
protobuf `FileDescriptorLegacy` class, which is documented as:

// TODO Remove this deprecated API entirely.

So we probably shouldn't rely on it.

Instead, this commit obtains the `FileDescriptorProto` corresponding
to the passed in `FieldDescriptor` and interrogates the `syntax` field
directly. This is a single implementation with no version-specific
workarounds. Hopefully this won't break in the next Google protobuf
release.

I tested the `FieldSyntax()` implementation in this commit across a
number of different Google protobuf releases and found that it worked
(`make && make check`) on all of them:

- Google protobuf 3.6.1.3 (Ubuntu 20.04)
- Google protobuf 3.12.4 (Ubuntu 22.04)
- Google protobuf 3.21.12 (Debian 12 + Debian unstable)
- Google protobuf 3.25.2 (Debian experimental)
- Google protobuf 26.1-dev

protoc-c/c_helpers.h

@@ -70,10 +70,6 @@
 #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 {
@@ -173,13 +169,21 @@ struct NameIndex
 int compare_name_indices_by_name(const void*, const void*);
 
 // Return the syntax version of the file containing the field.
-// This wrapper is needed to be able to compile against protobuf2.
 inline int FieldSyntax(const FieldDescriptor* field) {
-#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
+  auto proto = FileDescriptorProto();
+  field->file()->CopyTo(&proto);
+
+  if (proto.has_syntax()) {
+    auto syntax = proto.syntax();
+    assert(syntax == "proto2" || syntax == "proto3");
+    if (syntax == "proto2") {
+      return 2;
+    } else if (syntax == "proto3") {
+      return 3;
+    }
+  }
+
+  return 2;
 }
 
 // Work around changes in protobuf >= 22.x without breaking compilation against