Split Level::DelayedBug in two.

The two kinds of delayed bug have quite different semantics so a
stronger conceptual separation is nice. (`is_error` is a good example,
because the two kinds have different behaviour.)

The commit also moves the `DelayedBug` variant after `Error` in `Level`,
to reflect the fact that it's weaker than `Error` -- it might trigger an
error but also might not. (The pre-existing `downgrade_to_delayed_bug`
function also reflects the notion that delayed bugs are lower/after
normal errors.)

Plus it condenses some of the comments on `Level` into a table, for
easier reading, and introduces `can_be_top_or_sub` to indicate which
levels can be used in top-level diagnostics vs. subdiagnostics.

Finally, it renames `DiagCtxtInner::span_delayed_bugs` as
`DiagCtxtInner::delayed_bugs`. The `span_` prefix is unnecessary because
some delayed bugs don't have a span.

compiler/rustc_errors/src/emitter.rs

@@ -2116,6 +2116,7 @@ impl HumanEmitter {
                 }
                 if !self.short_message {
                     for child in children {
+                        assert!(child.level.can_be_top_or_sub().1);
                         let span = &child.span;
                         if let Err(err) = self.emit_messages_default_inner(
                             span,