Sean Baxter's circle compiler uses LLVM as a backend, but I believe the rest is from scratch.

Arguably these days having a clear frontend/backend separation is good compiler architecture. It might slow down compile times a bit, but it's worth the cost.

So it sounds like he wrote the frontend of a cpp compiler? There's a lot of work in other layers as well.

It wouldn't have made much sense to write the preprocessor these days, too, but it is part of the C++ compiler. Unless integrating it with the C++ lexer for speed purposes, as I did.