Fix a error suggestion for E0121 when using placeholder _ as return types on function signature.
Recommit after refactoring based on comment:
https://github.com/rust-lang/rust/pull/126017#issuecomment-2189149361
But when changing return type's lifetime to `ReError` will affect the subsequent borrow check process and cause test11 in typeck_type_placeholder_item.rs to lost E0515 message.
```rust
fn test11(x: &usize) -> &_ {
//~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
&x //~ ERROR cannot return reference to function parameter(this E0515 msg will disappear)
}
```
fixes#125488
r? ``@pnkfelix``
Recommit after refactoring based on comment:
https://github.com/rust-lang/rust/pull/126017#issuecomment-2189149361
But when changing return type's lifetime to `ReError` will affect the subsequent borrow check process and cause test11 in typeck_type_placeholder_item.rs to lost E0515 message.
```rust
fn test11(x: &usize) -> &_ {
//~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
&x //~ ERROR cannot return reference to function parameter(this E0515 msg will disappear)
}
```
