Rollup merge of #138482 - nnethercote:fix-hir-printing, r=compiler-errors

Fix HIR printing of parameters

HIR pretty printing does the wrong thing for anonymous parameters, and there is no test coverage for it. This PR remedies both of those things.

r? ``@lcnr``

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

@@ -1998,7 +1998,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 .iter()
                 .enumerate()
                 .map(|(i, ident)| {
-                    if ident.name.is_empty() || ident.name == kw::SelfLower {
+                    if ident.name.is_empty()
+                        || ident.name == kw::Underscore
+                        || ident.name == kw::SelfLower
+                    {
                         format!("arg{i}")
                     } else {
                         format!("{ident}")