Rollup merge of #80284 - ThePuzzlemaker:issue-80179-fix, r=varkor

Suggest fn ptr rather than fn item and suggest to use `Fn` trait bounds rather than the unique closure type in E0121

Previously, using `_` as a return type in a function that returned a function/closure would provide a diagnostic that would cause a papercut. For example:
```rust
fn f() -> i32 { 0 }
fn fn_ptr() -> _ { f }
fn closure() -> _ { || 0 }
```
would result in this diagnostic:
```rust
error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> <anon>:2:16
  |
2 | fn fn_ptr() -> _ { f }
  |                ^
  |                |
  |                not allowed in type signatures
  |                help: replace with the correct return type: `fn() -> i32 {f}`

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> <anon>:3:17
  |
3 | fn closure() -> _ { || 0 }
  |                 ^
  |                 |
  |                 not allowed in type signatures
  |                 help: replace with the correct return type: `[closure@<anon>:3:21: 3:25]`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0121`.
```
As can be seen, it was suggested to use the function definition return type `fn() -> i32 { f }` which is not valid syntax as a return type. Additionally, closures cause a papercut as unique closure types (notated in this case as `[closure@<anon>:3:21: 3:25]`) are not valid syntax either.

Instead, this PR implements this version of the diagnostic (this example is for the same code featured above):
```rust
error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> <anon>:2:16
  |
2 | fn fn_ptr() -> _ { f }
  |                ^
  |                |
  |                not allowed in type signatures
  |                help: replace with the correct return type: `fn() -> i32`

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> <anon>:3:17
  |
3 | fn closure() -> _ { || 0 }
  |                 ^ not allowed in type signatures
  |
  = help: consider using an `Fn`, `FnMut`, or `FnOnce` trait bound
  = note: for more information on `Fn` traits and closure types, see https://doc.rust-lang.org/book/ch13-01-closures.html

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0121`.
```
As can be seen in this diagnostic, the papercut for returning a function item is fixed by suggesting the usage of a function pointer as the return type. As for closures, it's suggested to use an `Fn`, `FnMut`, or `FnOnce` trait bound (with further reading on closures and `Fn` traits in *The Book* for beginners). I did not implement a suggestion to use `impl Fn() -> i32` syntax as that was out-of-scope for my abilities at the moment, therefore someone in the future may want to implement that. Also, it's possible to use either `impl Trait` syntax, generics, or generics with a `where` clause, and some users may not want to use `impl Trait` syntax for their own reasons.

This PR fixes #80179.

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