[clangd] Type hierarchy: don't resolve parents if the client only asked for children

Summary: Also reorganize the code for computing supertypes to make it more symmetric to subtypes.

Reviewers: kadircet

Reviewed By: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64613

llvm-svn: 366338