[clangd] Introduce loading of shards within auto-index

Summary:
Whenever a change happens on a CDB, load shards associated with that
CDB before issuing re-index actions.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D55224

llvm-svn: 350847