Set up demo C++ project with precise code intelligence

[varungandhi-src]

If we can showcase how some of the complex features in C++ work with precise code intel, that would be compelling to people who are interested in setting up lsif-clang on their codebase.

We should try to find a C++ project that is:

• Easy to build
• Doesn't require updating dependencies often (so less maintenance for us)
• Has a low commit rate (so we're not wasting CI minutes)

which can serve as a demo for lsif-clang.

It should probably also be C++11 or C++14, since the LLVM we forked is relatively old.

[tjdevries]

Could also just fork a project and then not commit to it anymore to use as a demo

[varungandhi-src]

Ouch, looks like we cite a bunch of examples in our docs (https://github.com/sourcegraph/lsif-clang/blob/main/docs/examples.md) but they don't work. 😬