Diagnostic should explain that 'impl trait' captures generic parameters

[Aaron1011]

The following code: playground:

trait MyTrait {}
impl MyTrait for bool {}

fn make_it<T>() -> impl MyTrait {
    true
}

fn main() {
    let mut a = make_it::<String>();
    a = make_it::<u8>();
}

produces the following error:

error[E0308]: mismatched types
  --> src/main.rs:10:9
   |
10 |     a = make_it::<u8>();
   |         ^^^^^^^^^^^^^^^ expected struct `std::string::String`, found u8
   |
   = note: expected type `impl MyTrait` (struct `std::string::String`)
              found type `impl MyTrait` (u8)

However, it does not explain where u8 and String come from.

When emitting a 'mistmatched type' error for two 'impl trait' types with the same DefId, we should explain that each set of generic parameters creates a unique opaque type, which is not equivalent to other opaque types generated by the same function.

See #66463 (comment) for the original discussion

[estebank]

Current output:

error[[E0308]](https://doc.rust-lang.org/stable/error_codes/E0308.html): mismatched types
  --> src/main.rs:10:9
   |
4  | fn make_it<T>() -> impl MyTrait {
   |                    ------------
   |                    |
   |                    the expected opaque type
   |                    the found opaque type
...
9  |     let mut a = make_it::<String>();
   |                 ------------------- expected due to this value
10 |     a = make_it::<u8>();
   |         ^^^^^^^^^^^^^^^ expected `String`, found `u8`
   |
   = note: expected opaque type `impl MyTrait` (`String`)
              found opaque type `impl MyTrait` (`u8`)
   = note: distinct uses of `impl Trait` result in different opaque types