Cranelift as a backend for clang

#### Feature

Since Cranelift is already experimentally working for rustc, I'd like to discuss if it could work for clang, and if so what would be involved. For instance, does [rustc_codegen_cranelift](https://github.com/rust-lang/rustc_codegen_cranelift) turn miri into Cranelift IR, or does it turn LLVM IR into Cranelift IR? From benchmarking my own clang builds, I can see that the backend is a bottleneck and I'd love to see it get faster.

@bjorn3