Commit 0bd9667 added bounds checking of our current target byte
position to prevent infinite loops. Unfortunately it was comparing the
file-relative `target` versus the global relative `file_start_pos` and
`file_end_pos`.

The result is failing to detect multibyte characters unless their
file-relative offset fit within their global offset. This causes other
parts of the compiler to generate spans pointing to the middle of a
multibyte character which will ultimately panic in
`bytepos_to_file_charpos`.

Fix by comparing the `target` to the total file size when moving forward
and doing checked subtraction when moving backwards. This should
preserve the intent of the bounds check while removing the offset
confusion.

Fixes #48508

This is named for the issue as it's testing the specific details of that
bug. It's a bit tricky as the ICE requires multiple files and debug info
enabled to trigger.

The out-of-line module #37195