[clangd] Don't create as much garbage while building Dex index.

Summary:
The Token objects are relatively expensive and we were spending a lot of
CPU creating them for each trigram emitted. Instead, use a tiny trigram
structure until we're ready to finalize the index.

This improves the new BuildDex benchmark by 20%. This code is hot and on
the critical path in clangd: it runs after a new preamble is built.

Reviewers: kbobyrev

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79918