[many_single_char_names]: Deduplicate diagnostics

[cookie-s]

Relates to #12379

Fix many_single_char_names lint so that it doesn't emit diagnostics when the current level of the scope doesn't contain any single character name.

let (a, b, c, d): (i32, i32, i32, i32);
match 1 {
  1 => (),
  e => {},
}

produced the exact same MANY_SINGLE_CHAR_NAMES diagnostic at each of the Arm e => {} and the Block {}.

---

changelog: [many_single_char_names]: Fix duplicate diagnostics

[rustbot]

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[Alexendoo]

Thank you!

@bors r+

[bors]

📌 Commit 7110f47 has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7110f47 with merge da4b212...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Alexendoo
Pushing da4b212 to master...