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_trait_selection/src/traits/const_evaluatable.rs

@@ -62,9 +62,10 @@ pub fn is_const_evaluatable<'tcx>(
 
         match unexpanded_ct.kind() {
             ty::ConstKind::Expr(_) => {
-                // FIXME(generic_const_exprs): we have a `ConstKind::Expr` which is fully concrete, but
-                // currently it is not possible to evaluate `ConstKind::Expr` so we are unable to tell if it
-                // is evaluatable or not. For now we just ICE until this is implemented.
+                // FIXME(generic_const_exprs): we have a `ConstKind::Expr` which is fully concrete,
+                // but currently it is not possible to evaluate `ConstKind::Expr` so we are unable
+                // to tell if it is evaluatable or not. For now we just ICE until this is
+                // implemented.
                 Err(NotConstEvaluatable::Error(tcx.dcx().span_delayed_bug(
                     span,
                     "evaluating `ConstKind::Expr` is not currently supported",