[clangd] Index Interfaces for Xrefs

Summary:
This is the first step of implementing Xrefs in clangd:
  - add index interfaces, and related data structures.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D49658

llvm-svn: 339011