c836c24994
This reverts #46722, commite0ab5d5feb. Since #111167, commit10b69dde3f, we are generating DWARF subprograms in a way that is meant to be more compatible with LLVM's expectations, so hopefully we don't need this workaround rewriting CUs anymore.
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.