message: Don't use an appendable file when dumping out LLVM IR.
  Appending to existing files is less ergonomic since we end up with invalid and
  confusing IR after multiple runs.
diff: @@ -363,7 +363,7 @@ llvm::CodeGenOpt::Level CodeGenOptLevel(const HloModuleConfig& module_config) {
  Status AppendIRToFile(const string& file_name, const string& ir_module_string) {
  std::unique_ptr<tensorflow::WritableFile> f;
  TF_RETURN_IF_ERROR(
  - tensorflow::Env::Default()->NewAppendableFile(file_name, &f));
  + tensorflow::Env::Default()->NewWritableFile(file_name, &f));
  TF_RETURN_IF_ERROR(f->Append(ir_module_string));
  TF_RETURN_IF_ERROR(f->Close());
  return Status::OK();
  