Method openFileForRead

src/sema_manager.cc:52–58

      VFS(IntrusiveRefCntPtr<llvm::vfs::FileSystem> fs, PreambleStatCache &cache)
          : ProxyFileSystem(std::move(fs)), cache(cache) {}
      llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>> openFileForRead(const Twine &path) override {
        auto file = getUnderlyingFS().openFileForRead(path);
        if (!file || !*file)
          return file;
        cache.update(path, file->get()->status());
        return file;
      }
      llvm::ErrorOr<llvm::vfs::Status> status(const Twine &path) override {
        auto s = getUnderlyingFS().status(path);
        cache.update(path, s);