[clangd] Type hierarchy subtypes

Summary:
This builds on the relations support added in D59407, D62459, D62471,
and D62839 to implement type hierarchy subtypes.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58880

llvm-svn: 363506