Change collect_and_partition_mono_items tuple return type to a struct

Author: oli-obk

Diff for: src/driver/aot.rs

@@ -676,7 +676,7 @@ pub(crate) fn run_aot(
     .to_owned();
 
     let cgus = if tcx.sess.opts.output_types.should_codegen() {
-        tcx.collect_and_partition_mono_items(()).1
+        tcx.collect_and_partition_mono_items(()).codegen_units
     } else {
         // If only `--emit metadata` is used, we shouldn't perform any codegen.
         // Also `tcx.collect_and_partition_mono_items` may panic in that case.