rustc_error: make ErrorReported impossible to construct

There are a few places were we have to construct it, though, and a few
places that are more invasive to change. To do this, we create a
constructor with a long obvious name.

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1396,7 +1396,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                 // `rustc_ty_utils::instance::resolve_associated_item()`.
                 let node_item =
                     assoc_def(selcx, impl_data.impl_def_id, obligation.predicate.item_def_id)
-                        .map_err(|ErrorGuaranteed| ())?;
+                        .map_err(|ErrorGuaranteed { .. }| ())?;
 
                 if node_item.is_final() {
                     // Non-specializable items are always projectable.