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)
}

```
6 files changed
tree: 81229a01ced8a58c2cb8195161352990c07ee6a9
  1. .github/
  2. .reuse/
  3. compiler/
  4. library/
  5. LICENSES/
  6. src/
  7. tests/
  8. .editorconfig
  9. .git-blame-ignore-revs
  10. .gitattributes
  11. .gitignore
  12. .gitmodules
  13. .mailmap
  14. Cargo.lock
  15. Cargo.toml
  16. CODE_OF_CONDUCT.md
  17. config.example.toml
  18. configure
  19. CONTRIBUTING.md
  20. COPYRIGHT
  21. INSTALL.md
  22. LICENSE-APACHE
  23. LICENSE-MIT
  24. README.md
  25. RELEASES.md
  26. rust-bors.toml
  27. rustfmt.toml
  28. triagebot.toml
  29. x
  30. x.ps1
  31. x.py
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