[clangd] Collect the number of files referencing a symbol in the static index.

Summary:
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