find diagnostic suggestion loops

[matthiaskrgr]

https://rust-lang.zulipchat.com/#narrow/stream/147480-t-compiler.2Fwg-diagnostics/topic/automatic.20diagnostic.20verification/near/313156325

if that gets you too many wontfixes, a looser thing you could check instead is that suggestions should converge when running cargo fix repeatedly.

for example, something has gone wrong if the suggestions loop between A <--> B, or diverge with A --> (A) --> ((A)) --> ...