[clangd] Add RelationSlab

Summary:
RelationSlab is a new index data structure that stores relations between
symbols.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59407

llvm-svn: 362352