Make unnecessary_fold commutative

[mikhailofff]

To be honest, the changes are confusing.

First of all, it has already been stated that it would be wrong if we extended the lint to cover both sides.

rust-clippy/tests/ui/unnecessary_fold.rs

Lines 91 to 95 in 18e5b2d

	// We only match against an accumulator on the left
	// hand side. We could lint for .sum and .product when
	// it's on the right, but don't for now (and this wouldn't
	// be valid if we extended the lint to cover arbitrary numeric
	// types).

Secondly, there is some monstrous expression in the get_triggered_expr_span function, and I think it could be rewritten more cleanly.

Anyway, even if it has to be implemented, I'm glad to rewrite it more thoroughly.

Fixes #16581.

---

changelog: [unnecessary_fold]: match against an accumulator on both sides.

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[llogiq]

It would only be invalid if we supported arbitrary numeric types. So I don't see this as a problem. We can always revert if it becomes one.

The code itself looks ok to me. Thank you!