Don't consider delayed bugs for `-Ztreat-err-as-bug`.

`-Ztreat-err-as-bug` treats normal errors and delayed bugs equally,
which can lead to some really surprising results.

This commit changes `-Ztreat-err-as-bug` so it ignores delayed bugs,
unless they get promoted to proper bugs and are printed.

This feels to me much simpler and more logical. And it simplifies the
implementation:
- The `-Ztreat-err-as-bug` check is removed from in
  `DiagCtxt::{delayed_bug,span_delayed_bug}`.
- `treat_err_as_bug` doesn't need to count delayed bugs.
- The `-Ztreat-err-as-bug` panic message is simpler, because it doesn't
  have to mention delayed bugs.

Output of delayed bugs is now more consistent. They're always printed
the same way. Previously when they triggered `-Ztreat-err-as-bug` they
would be printed slightly differently, via `span_bug` in
`span_delayed_bug` or `delayed_bug`.

A minor behaviour change: the "no errors encountered even though
`span_delayed_bug` issued" printed before delayed bugs is now a note
rather than a bug. This is done so it doesn't get counted as an error
that might trigger `-Ztreat-err-as-bug`, which would be silly.
This means that if you use `-Ztreat-err-as-bug=1` and there are no
normal errors but there are delayed bugs, the first delayed bug will be
shown (and the panic will happen after it's printed).

Also, I have added a second note saying "those delayed bugs will now be
shown as internal compiler errors". I think this makes it clearer what
is happening, because the whole concept of delayed bugs is non-obvious.

There are some test changes.
- equality-in-canonical-query.rs: Minor output changes, and the error
  count reduces by one because the "no errors encountered even though
  `span_delayed_bug` issued" message is no longer counted as an error.
- rpit_tait_equality_in_canonical_query.rs: Ditto.
- storage-live.rs: The query stack disappears because these delayed bugs
  are now printed at the end, rather than when they are created.
- storage-return.rs, span_delayed_bug.rs: now need
  `-Zeagerly-emit-delayed-bugs` because they need the delayed bugs
  emitted immediately to preserve behaviour.

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