Fix platform-specific stderr mismatch in ice_contract_attr_on_eii_generated_item test

The previous test used `fn implementation() {}` with a body, which caused
`generate_default_impl` to generate a `const _: () = { fn implementation() {} }`
item containing `self::implementation`. On Linux (aarch64-gnu-llvm-21), the
resolver's `suggest_ident_hidden_by_hygiene` emitted an extra help span on the
resulting E0425 error that did not appear on macOS, causing a stderr mismatch.

Switch the declaration to `fn implementation();` (no body) so that
`generate_default_impl` is not called and no `self::implementation` path is
emitted. The test still validates that `#[eii]` + `#[core::contracts::ensures]`
produces graceful errors instead of an ICE, via the two contract-annotation
errors on the generated foreign item.

Author: GokhanKabar

tests/ui/eii/ice_contract_attr_on_eii_generated_item.rs

@@ -2,11 +2,10 @@
 
 #![feature(extern_item_impls)]
 #![feature(contracts)]
-//~^ WARN the feature `contracts` is incomplete
+#![allow(incomplete_features)]
 
 #[eii]
 #[core::contracts::ensures]
 //~^ ERROR contract annotations is only supported in functions with bodies
 //~| ERROR contract annotations can only be used on functions
-fn implementation() {}
-//~^ ERROR cannot find value `implementation` in module `self`
+fn implementation();

tests/ui/eii/ice_contract_attr_on_eii_generated_item.stderr

@@ -10,21 +10,5 @@ error: contract annotations can only be used on functions
 LL | #[core::contracts::ensures]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0425]: cannot find value `implementation` in module `self`
-  --> $DIR/ice_contract_attr_on_eii_generated_item.rs:11:4
-   |
-LL | fn implementation() {}
-   |    ^^^^^^^^^^^^^^ not found in `self`
-
-warning: the feature `contracts` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/ice_contract_attr_on_eii_generated_item.rs:4:12
-   |
-LL | #![feature(contracts)]
-   |            ^^^^^^^^^
-   |
-   = note: see issue #128044 <https://github.com/rust-lang/rust/issues/128044> for more information
-   = note: `#[warn(incomplete_features)]` on by default
-
-error: aborting due to 3 previous errors; 1 warning emitted
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0425`.