[clangd] Index API and implementations for relations

Summary:
This builds on the relations support added in D59407, D62459,
and D62471 to expose relations in SymbolIndex and its
implementations.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62839

llvm-svn: 363481