Use fewer delayed bugs.

For some cases where it's clear that an error has already occurred,
e.g.:
- there's a comment stating exactly that, or
- things like HIR lowering, where we are lowering an error kind

The commit also tweaks some comments around delayed bug sites.

compiler/rustc_hir_analysis/src/coherence/unsafety.rs

@@ -85,7 +85,7 @@ pub(super) fn check_item(
 
         (_, _, Unsafety::Unsafe, Negative) => {
             // Reported in AST validation
-            tcx.dcx().span_delayed_bug(tcx.def_span(def_id), "unsafe negative impl");
+            assert!(tcx.dcx().has_errors().is_some(), "unsafe negative impl");
             Ok(())
         }
         (_, _, Unsafety::Normal, Negative)