[clangd] Migrate Lexer usages in TypeHierarchy to TokenBuffers

Summary:
Also fixes a bug, resulting from directly using ND.getEndLoc() for end
location of the range. As ND.getEndLoc() points to the begining of the last
token, whereas it should point one past the end, since LSP ranges are half open
(exclusive on the end).

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74850