Does it depend on clang? Because LLVM can't compile C by itself.

So ? The fact that rustc compiler uses a component written in C++ doesn't mean that even pure Rust projects have a C++ dependency. Follow any language's bootstrap chain and you'll find C somewhere (or a bootstrapping purists telling you that didn't properly bootstrap the thing), making the "some of the FOOLANG compiler is written in BARLANG" argument pointless.

You don't need a C or C++ compiler (not even Clang) to compile Arti. The rustc packages typically use a vendored version of LLVM.