[NLL] Loss of suggestion to introduce a named lifetime parameter

[marmeladema]

In the src/test/ui/self/arbitrary_self_types_pin_lifetime_mismatch.base.rs test, we give the suggestion to consider introducing a named lifetime parameter and update trait if needed under migrate mode. Under NLL, the error message is imo clearer but we lose that suggestion.

[marmeladema]

@rustbot label +A-NLL +NLL-diagnostics