Rollup merge of #104593 - compiler-errors:rpitit-object-safety-spans, r=fee1-dead

Improve spans for RPITIT object-safety errors

No reason why we can't point at the `impl Trait` that causes the object-safety violation.

Also [drive-by: Add is_async fn to hir::IsAsync](c4165f3a96), which touches clippy too.

compiler/rustc_hir_analysis/src/check/compare_method.rs

@@ -684,9 +684,7 @@ fn report_trait_method_mismatch<'tcx>(
                 // Suggestion to change output type. We do not suggest in `async` functions
                 // to avoid complex logic or incorrect output.
                 match tcx.hir().expect_impl_item(impl_m.def_id.expect_local()).kind {
-                    ImplItemKind::Fn(ref sig, _)
-                        if sig.header.asyncness == hir::IsAsync::NotAsync =>
-                    {
+                    ImplItemKind::Fn(ref sig, _) if !sig.header.asyncness.is_async() => {
                         let msg = "change the output type to match the trait";
                         let ap = Applicability::MachineApplicable;
                         match sig.decl.output {