Fix while_let_on_iterator dropping loop label when applying fix.

[jusexton]

Loop label was not persisted when displaying help and was therefore producing broken rust code when applying fixes.

Solution was to store the ast::Label when creating a higher::WhileLet from an expression and add the label name to the lint suggestion and diagnostics.

---

Fixes: #13123

changelog: [while_let_on_iterator]: Fix issue dropping loop label when displaying help and applying fixes.

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[dswij]

LGTM, thanks!

[dswij]

@bors r+

[bors]

📌 Commit 0cdf6e1 has been approved by dswij

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0cdf6e1 with merge d20be39...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: dswij
Pushing d20be39 to master...