CI testing against real-world projects

[varungandhi-src]

A rough initial shortlist of projects based on the awesome-cpp list:

• https://github.com/Tencent/rapidjson
• https://github.com/simdjson/simdjson
• https://github.com/beltoforion/muparser
• https://github.com/Qihoo360/evpp
• https://github.com/intel/hyperscan
• https://github.com/kkos/oniguruma
• https://github.com/ThePhD/sol2
• https://github.com/capnproto/capnproto
• https://github.com/CrowCpp/Crow
• https://github.com/zeux/pugixml
• https://github.com/jbeder/yaml-cpp

I don't know how big each of these are, but I've deliberately avoided picking very large projects (e.g. torvalds/linux).

The idea is that we'd set up a image with Ubuntu 22.04, build lsif-clang from source, and index each of these using Buildkite, preferably on a high core count machine.

This work should largely be reusable for any future indexers as well.