[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