Detect type parameters that were meant to be `impl Trait` in return position

[estebank]

Given fn foo<T: Any>() -> T { "" } the output is:

error[E0308]: mismatched types
 --> src/main.rs:3:5
  |
2 | fn foo<T: Any>() -> T {
  |        -            - expected `T` because of return type
  |        |
  |        this type parameter
3 |     ""
  |     ^^ expected type parameter `T`, found `&str`
  |
  = note: expected type parameter `T`
                  found reference `&'static str`

We should be suggesting the use of impl Trait while explaining that the callers of foo will only be able to access whatever the trait bounds allow: fn foo() -> impl Any { "" }. We should also likely add an additional note for the special case of an Any bound letting people know that it is very unlikely what they want to do.

(Taken from a subset of https://users.rust-lang.org/t/how-can-we-return-a-anonymous-type-in-result-trait/51258)

[estebank]

Fixed:

error[E0308]: mismatched types
 --> src/main.rs:3:5
  |
2 | fn foo<T: Any>() -> T {
  |        -            -
  |        |            |
  |        |            expected `T` because of return type
  |        |            help: consider using an impl return type: `impl Any`
  |        this type parameter
3 |     ""
  |     ^^ expected type parameter `T`, found `&str`
  |
  = note: expected type parameter `T`
                  found reference `&'static str`