message: fix a file path problem on windows
diff: #include "verilog/preprocessor/verilog_preprocess.h"
  +#include <filesystem>
  #include <functional>
  #include <iterator>
  #include <memory>
  @@ -568,16 +569,17 @@ absl::Status VerilogPreprocess::HandleInclude(
  }
  // Currently the file path looks like "path", we need to remove "".
  const auto& token_text = file_token_iter->text();
  - absl::string_view file_path(token_text.begin() + 1, token_text.size() - 2);
  - bool is_relative = file_path.find_first_of("/");
  + std::filesystem::path file_path =
  + std::string(token_text.substr(1, token_text.size() - 2));
  - std::string file_path_to_search = std::string(file_path);
  + std::string file_path_to_search = file_path.string();
  - if (is_relative) {
  + if (file_path.is_relative()) {
  bool found = false;
  for (const auto& base_dir : preprocess_info_.include_dirs) {
  - file_path_to_search = absl::StrCat(base_dir, "/", file_path);
  + file_path_to_search =
  + verible::file::JoinPath(base_dir, file_path.string());
  auto search_status = verible::file::FileExists(file_path_to_search);
  if (search_status.ok()) {
  found = true;
  