[redundant_guards]: catch is_empty, starts_with and ends_with on slices and strs

[y21]

Fixes #11807

Few things worth mentioning:

• Taking snippets is now done at callsite, instead of passing a span and doing it in emit_redundant_guards. This is because we now need custom suggestion strings in certain places, like "" for str::is_empty.
• This now uses snippet instead of snippet_with_applicability. I don't think this really makes any difference for MaybeIncorrect, though?
• This could also lint byte strings, as they're of type &[u8; N], but that can be ugly so I decided to leave it out for now

changelog: [redundant_guards]: catch str::is_empty, slice::is_empty, slice::starts_with and slice::ends_with

[rustbot]

r? @llogiq

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[llogiq]

This looks good to me. Thank you!

@bors r+

[bors]

📌 Commit 8f9c738 has been approved by llogiq

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8f9c738 with merge 8b0bf64...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 8b0bf64 to master...