fix: Fix postfix snippets duplicating derefs

[ChayimFriedman2]

Fixes #19758.

I'm not entirely sure the found_ref_or_deref check is correct - I think maybe we should remove it - but removing it makes let postfix completions no longer appear (although I suspect this is because we check the grandparent instead of the parent).

[Veykril]

I think the proper fix here is to also consider the type adjustments of the receiver. As dbg! for example actually needs make the adjustments (where possible) explicit to be completely correct if I am not mistaken?

[ChayimFriedman2]

Adjustments depend on the method invoked, so we cannot know them.

[Veykril]

Oh right, sorry, I meant the adjustments of a parent field access / method call expression in cases like x.$0.foo() -> dbg!(x).foo(), here we need to force the adjustments into the dbg! call to be entirely correct.

But that's a separate concern, mixed up the problems here

[ChayimFriedman2]

I think this will be pretty hard to achieve because inference is done on expr.MISSING.field, and MISSING isn't a field/method of expr.

[Veykril]

Ye that would require even more speculative execution I think 😬