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_ast_lowering/src/expr.rs

@@ -323,9 +323,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     )
                 }
                 ExprKind::Yield(opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
-                ExprKind::Err => {
-                    hir::ExprKind::Err(self.dcx().span_delayed_bug(e.span, "lowered ExprKind::Err"))
-                }
+                ExprKind::Err => hir::ExprKind::Err(self.dcx().has_errors().unwrap()),
                 ExprKind::Try(sub_expr) => self.lower_expr_try(e.span, sub_expr),
 
                 ExprKind::Paren(_) | ExprKind::ForLoop { .. } => {