Rollup merge of #81914 - kper:fixing-81885, r=estebank

Rollup merge of #81914 - kper:fixing-81885, r=estebank

Fixing bad suggestion for `_` in `const` type when a function #81885

Closes #81885

```
error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> $DIR/typeck_type_placeholder_item_help.rs:13:22
   |
LL | const TEST4: fn() -> _ = 42;
   |                      ^
   |                      |
   |                      not allowed in type signatures
   |                      help: use type parameters instead: `T`
```

Do not show the suggestion `help: use type parameters instead: T` when `fn`