codeintel: Add denormalized reference counts to uploads
Created by: efritz
This PR adds a field to track the number of references to an upload. This PR also introduces a pair of dbstore methods that can calculate the value of this field for a single upload, and modify (increment/decrement) the counts of an upload's direct dependencies. This methods are not used.
Upcoming PRs will:
- Ensure that this value changes as records are created
- #24612
- Ensure that this value changes as records are deleted
- #24613
- Backfill the correct value to existing records
- #24611