[clangd] Support typeHierarchy/resolve for children of parents as well

Summary:
The initial implementation of typeHierarchy/resolve only supported
cases where an initial request was made for children, and then
typeHierarchy/resolve was used to get additional levels of children.

However, a client may also want to make an initial request for
parents, and then show other children of those parents, so support
typeHierarchy/resolve for items returned in response to a request
for parents as well.

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81845