minor: Simplify

fix span calculation for non-ascii in `needless_return`

Fixes rust-lang#12491

Probably fixes rust-lang#12328 as well, but that one has no reproducer, so 🤷

The bug was that the lint used `rfind()` for finding the byte index of the start of the previous non-whitespace character:
```
// abc\n     return;
     ^
```
... then subtracting one to get the byte index of the actual whitespace (the `\n` here).
(Subtracting instead of adding because it treats this as the length from the `return` token to the `\n`)

That's correct for ascii, like here, and will get us to the `\n`, however for non ascii, the `c` could be multiple bytes wide, which would put us in the middle of a codepoint if we simply subtract 1 and is what caused the ICE.

There's probably a lot of ways we could fix this.
This PR changes it to iterate backwards using bytes instead of characters, so that when `rposition()` finally finds a non-whitespace byte, we *know* that we've skipped exactly 1 byte. This was *probably*(?) what the code was intending to do

changelog: Fix ICE in [`needless_return`] when previous line end in a non-ascii character