Rollup merge of #102763 - compiler-errors:nits, r=cjgillot

Some diagnostic-related nits

1. Use `&mut Diagnostic` instead of `&mut DiagnosticBuilder<'_, T>`
2. Make `diag.span_suggestions` take an `IntoIterator` instead of `Iterator`, just to remove some `.into_iter` calls on the caller.

idk if I should add a lint to make sure people use `&mut Diagnostic` instead of `&mut DiagnosticBuilder<'_, T>` in cases where we're just, e.g., adding subdiagnostics to the diagnostic... maybe a followup.

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1116,7 +1116,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         err.span_suggestions(
                             span.shrink_to_lo(),
                             "consider borrowing here",
-                            ["&".to_string(), "&mut ".to_string()].into_iter(),
+                            ["&".to_string(), "&mut ".to_string()],
                             Applicability::MaybeIncorrect,
                         );
                     } else {