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.

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