Rollup merge of #48522 - etaoins:fix-find-width-of-character-at-span-bounds-check, r=estebank

Fix find_width_of_character_at_span bounds check

Commit 0bd96671f0 added bounds checking of our current target byte position to prevent infinite loops. Unfortunately it was comparing the file-relative `target` versus the global `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.

cc @davidtwco

Fixes #48508

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