This PR changes the "declaration uses 'sorry'" warning to use backticks
instead of single quotes, consistent with Lean's conventions for
formatting code identifiers in diagnostic messages.
This PR adds a `warn.sorry` option (default true) that logs the
"declaration uses 'sorry'" warning when declarations contain `sorryAx`.
When false, the warning is not logged.
Closes#8611 (assuming that one would set `warn.sorry` as an extra flag
when building).
Other change: Uses `warn.sorry` when creating auxiliary declarations in
`structure` elaborator, to suppress irrelevant 'sorry' warnings.
We could include the sorries themselves in the message if they are
labeled, letting users "go to definition" to see where the sorries are
coming from.
In an earlier version, added additional information to the warning when
it is a synthetic sorry, since these can be caused by elaboration bugs
and they can also be caused by elaboration failures in previous
declarations. This idea needs some more work, so it's not included.
