fix span calculation for non-ascii in needless_return

[y21]

Fixes #12491

Probably fixes #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

[rustbot]

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[Alexendoo]

Yeah they're probably the same issue, byte wise seems like a nice way of handling it. Thanks!

@bors r+

[bors]

📌 Commit d3f8f3e has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

⌛ Testing commit d3f8f3e with merge c9f2482...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Alexendoo
Pushing c9f2482 to master...