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_mir_transform/src/lib.rs

@@ -265,7 +265,7 @@ fn mir_const_qualif(tcx: TyCtxt<'_>, def: LocalDefId) -> ConstQualifs {
     let body = &tcx.mir_const(def).borrow();
 
     if body.return_ty().references_error() {
-        tcx.dcx().span_delayed_bug(body.span, "mir_const_qualif: MIR had errors");
+        assert!(tcx.dcx().has_errors().is_some(), "mir_const_qualif: MIR had errors");
         return Default::default();
     }