Functions and closures with incorrect return types should have better diagnostics

[Mark-Simulacrum]

We should do better on the diagnostic in this case, since presumably the author of the code meant to return the 10, so we should suggest that the function needs a return type, somewhat as we currently give the "remove ;" suggestion.

fn foo() {
   10
}

error[E0308]: mismatched types
 --> test.rs:2:5
  |
2 |     10
  |     ^^ expected (), found integral variable
  |
  = note: expected type `()`
             found type `{integer}`

error: aborting due to previous error

Filing this to close out #35478, #21838, #22216, #18595 as duplicates of this.

[estebank]

Previous stab at this: #39231.

[estebank]

We do this now.