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 upstreams NatCast and IntCast alone independent of norm_cast in
#3322.
This will allow more efficiently upstreaming parts of Std.Data.Int
relevant for omega.
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Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
