c2b15a6b64
And report an error if parsing the additional pass pipeline fails. Threading through the error accounts for most of the changes here.
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.