[clangd] Collect the number of files referencing a symbol in the static index.
This is an important ranking signal.
It's off for the dynamic index for now. Correspondingly, tell the index
infrastructure only to report declarations for the dynamic index.
Reviewers: ioeric, hokein
Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits
Differential Revision: https://reviews.llvm.org/D44315