[ObjC] Fix a clang tidy warning about const usage and make the FileGenerator const.

PiperOrigin-RevId: 491512974

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

@@ -205,7 +205,7 @@ FileGenerator::CommonState::CollectMinimalFileDepsContainingExtensionsInternal(
 // There are comments about what the expected code should be line and limited
 // testing objectivec/Tests/GPBUnittestProtos2.m around compilation (#imports
 // specifically).
-const std::vector<const FileDescriptor*>
+std::vector<const FileDescriptor*>
 FileGenerator::CommonState::CollectMinimalFileDepsContainingExtensions(
     const FileDescriptor* file) {
   absl::flat_hash_set<const FileDescriptor*> min_deps =

src/google/protobuf/compiler/objectivec/file.h

@@ -56,7 +56,7 @@ class FileGenerator {
   struct CommonState {
     CommonState() = default;
 
-    const std::vector<const FileDescriptor*>
+    std::vector<const FileDescriptor*>
     CollectMinimalFileDepsContainingExtensions(const FileDescriptor* file);
 
    private:

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

@@ -285,7 +285,7 @@ bool ObjectiveCGenerator::GenerateAll(
   FileGenerator::CommonState state;
   for (int i = 0; i < files.size(); i++) {
     const FileDescriptor* file = files[i];
-    FileGenerator file_generator(file, generation_options, state);
+    const FileGenerator file_generator(file, generation_options, state);
     std::string filepath = FilePath(file);
 
     // Generate header.