[clangd] Partition include graph on auto-index.

Summary:
Partitions include graphs in auto-index so that each shards contains
only part of the include graph related to itself.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D55062

llvm-svn: 348252