fix: manual_memcpy wrong indexing for multi dimensional arrays

[granddaifuku]

fixes: #9334

This PR fixes an invalid suggestion for multi-dimensional arrays.

For example,

let src = vec![vec![0; 5]; 5];
let mut dst = vec![0; 5];

for i in 0..5 {
    dst[i] = src[i][i];
}

For the above code, Clippy suggests dst.copy_from_slice(&src[i]);, but it is not compilable because i is only used to loop the array.
I adjusted it so that Clippy manual_memcpy works properly for multi-dimensional arrays.

changelog: [manual_memcpy]: Fixes invalid indexing suggestions for multi-dimensional arrays

[rustbot]

r? @Alexendoo

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[bors]

☔ The latest upstream changes (presumably #12198) made this pull request unmergeable. Please resolve the merge conflicts.

[granddaifuku]

Hi @Alexendoo , this is just a friendly reminder about this PR.
It would be appreciated if you could review it when you have time?
Thak you!

[bors]

☔ The latest upstream changes (presumably #12306) made this pull request unmergeable. Please resolve the merge conflicts.

[Alexendoo]

Thank you!

@bors r+

[bors]

📌 Commit dfedadc has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

⌛ Testing commit dfedadc with merge e865dca...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Alexendoo
Pushing e865dca to master...