b37c214ec2
Using an enum here was historically not worth the extra hassle, but now we can lean on `#[derive(TryFromU32)]` to hide most of the boilerplate.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.