(You know this, but for anyone else reading, don't forget that that doing this compiles LLVM, which is in of itself a massive C++ project, as well as compiling the compiler itself multiple times. That's part of why it would be such an outlier.)
Sometimes it is just that someone drank the Kool-aid and made everything a template. Then split their code across hundreds of tiny files and did the one big massive include at the top of every file that pulls in everything.
