Guard against overflow in `codemap::span_to_lines`.
Make `span_to_lines` to return a `Result`.
(This is better than just asserting internally, since it allows caller
to decide if they can recover from the problem.)
Added type alias for `FileLinesResult` returned by `span_to_lines`.
Update embedded unit test to reflect `span_to_lines` signature change.
In diagnostic, catch `Err` from `span_to_lines` and print
`"(internal compiler error: unprintable span)"` instead.
----
There a number of recent issues that report the bug here. See
e.g. #24761 and #24954.
This change *might* fix them. However, that is not its main goal.
The main goals are:
1. Make it possible for callers to recover from an error here, and
2. Insert a more conservative check, in that we are
also checking that the files match up.