[clangd] SymbolCollector support for relations

Summary:
The only relation currently collected is RelationBaseOf, because this is
all we need for type hierarchy subtypes. Additional relations can be
collected in the future as the need arises.

This patch builds on D59407 and D62459.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62471

llvm-svn: 362467