iirc, plain linux cannot be compiled without gcc. pretty sure there's this project (which, for the life of me, i can't remember the namełthat makes it possible to do so. so, does chimaera uses that project's work?
In addition to what reocha said, ClangBuiltLinux is the project you're probably thinking of. They track issues building with clang and provide CI: https://clangbuiltlinux.github.io/
That used to be true, but the work to compile Linux with clang was mainlined a while ago now. And IIRC Google builds their kernels with it (including Android and ChromeOS) so it's definitely getting used/tested.