[clangd] Remove TokenBuffer usage in TypeHierarchy

Summary:
This patch mostly reverts D74850.
We could not use `AST.getTokens()` here, because it does not have tokens from the preamble.

Reviewers: sammccall, kadircet

Reviewed By: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84144