[clangd] Populate include graph during static indexing action.

Summary:
This is the second part for introducing include hierarchy into index
files produced by clangd. You can see the base patch that introduces structures
and discusses the future of the patches in D54817

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D54999

llvm-svn: 348005