[mut_mut]: Fix duplicate diags

[cookie-s]

Relates to #12379

The mut_mut lint produced two diagnostics for each mut mut pattern in ty inside blocks because MutVisitor::visit_ty was called from MutMut::check_ty and MutMut::check_block independently. This PR fixes the issue.

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changelog: [mut_mut]: Fix duplicate diagnostics

[rustbot]

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[y21]

Looks good, thanks!

For something like &mut &mut &mut i32 I believe this lint also emits two warnings when it could just be one, but that's another issue unrelated to this one.

@bors r+

[bors]

📌 Commit 4e95b4a has been approved by y21

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4e95b4a with merge b2f9c4c...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: y21
Pushing b2f9c4c to master...