fn prepare_lto(
cgcx: &CodegenContext<LlvmCodegenBackend>,
exported_symbols_for_lto: &[String],
each_linked_rlib_for_lto: &[PathBuf],
dcx: DiagCtxtHandle<'_>,
) -> Result<(Vec<CString>, Vec<(SerializedModule<ModuleBuffer>, CString)>), FatalError>