[clangd] Fix updated file detection logic in indexing

Summary:
Files without any symbols were never marked as updated during indexing, which resulted in failure while writing shards for these files.

This patch fixes the logic to mark files that are seen for the first time but don't contain any symbols as updated.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D56592

llvm-svn: 351170