Fix non-existent-field ICE for generic fields.
I mentioned this ICE in a chat and it took about 3 milliseconds before `@eddyb` found the problem and said this change would fix it. :)
This also changes one the field types in the related test to one that triggered the ICE.
Fixes#81627.
Fixes#81672.
Fixes#81709.
Cc https://github.com/rust-lang/rust/pull/81480 `@b-naber` `@estebank.`