[clangd] Implement typeHierarchy/resolve for subtypes

[clangd] Implement typeHierarchy/resolve for subtypes

Summary:
This allows the client to resolve subtypes one level at a time.

For supertypes, this is not necessary, because we eagerly compute
supertypes and return all levels.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64308

llvm-svn: 365867