Dead-code pass highlights too much of impl functions

[mibac138]

Fixes #66627.
Previous diagnostic:

error: method is never used: `unused_impl_fn_3`
  --> src/main.rs:28:5
   |
28 | /     fn unused_impl_fn_3(
29 | |         var: i32,
30 | |     ) {
31 | |         println!("bar {}", var);
32 | |     }
   | |_____^

New diagnostic:

error: associated function is never used: `unused_impl_fn_3`
  --> $DIR/lint-dead-code-6.rs:13:8
   |
LL |     fn unused_impl_fn_3(
   |        ^^^^^^^^^^^^^^^^

This makes associated functions in line with free-standing functions.

[rust-highfive]

r? @cramertj

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[cramertj]

@bors r+

[bors]

📌 Commit b6d5d1f has been approved by cramertj