ignore the `use_existential_projection_new_instead` field in the `Debug` impl

[lcnr]

e.g. in #152789 (comment) we print ExistentialProjection { def_id: DefId(0:5 ~ foo[b507]::Trait::Assoc), args: [i32], term: Term::Ty(i64), use_existential_projection_new_instead: () }

We should not print the use_existential_projection_new_instead: ()

[lcnr]

Please copy the way this is done for _use_alias_ty_new_instead of ALiasTy