[ptr_arg]: recognize methods that also exist on slices

[y21]

Fixes #11816

Not a new lint, just a very small improvement to the existing ptr_arg lint which would have caught the linked issue.

The problem was that the lint checks if a Vec-specific method was called, that is, if the receiver is Vec<_>.
This is the case for len and is_empty, however these methods also exist on slices so we can still lint there.
This logic exists in a different lint, so we can just reuse that here.

Interestingly, there was even a comment up top that explained what it should have been doing, but the logic for it just wasn't there?

changelog: [ptr_arg]: recognize methods that also exist on slices

_{Also, this is my 100th PR to clippy 🎉}

[rustbot]

r? @Alexendoo

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[Alexendoo]

Nice fix, I wonder why those methods are duplicated

@bors r+

[bors]

📌 Commit 3e83a52 has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3e83a52 with merge 003e910...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Alexendoo
Pushing 003e910 to master...