Make cast suggestions verbose

```
error[E0604]: only `u8` can be cast as `char`, not `u32`
  --> $DIR/E0604.rs:2:5
   |
LL |     1u32 as char;
   |     ^^^^^^^^^^^^ invalid cast
   |
help: try `char::from_u32` instead
   |
LL -     1u32 as char;
LL +     char::from_u32(1u32);
   |
```

```
error[E0620]: cast to unsized type: `&[u8]` as `[char]`
  --> $DIR/cast-to-slice.rs:6:5
   |
LL |     arr as [char];
   |     ^^^^^^^^^^^^^
   |
help: try casting to a reference instead
   |
LL |     arr as &[char];
   |            +
```

```
error[E0620]: cast to unsized type: `Box<{integer}>` as `dyn Send`
  --> $DIR/cast-to-unsized-trait-object-suggestion.rs:3:5
   |
LL |     Box::new(1) as dyn Send;
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   |
help: you can cast to a `Box` instead
   |
LL |     Box::new(1) as Box<dyn Send>;
   |                    ++++        +
```

2 files changed, 13 insertions, 7 deletions

diff --git a/tests/ui/error-codes/E0604.stderr b/tests/ui/error-codes/E0604.stderr
index e91f74d6b3f..b949ed973cd 100644
--- a/tests/ui/error-codes/E0604.stderr
+++ b/tests/ui/error-codes/E0604.stderr
@@ -2,10 +2,13 @@ error[E0604]: only `u8` can be cast as `char`, not `u32`
   --> $DIR/E0604.rs:2:5
    |
 LL |     1u32 as char;
-   |     ^^^^^^^^^^^^
-   |     |
-   |     invalid cast
-   |     help: try `char::from_u32` instead: `char::from_u32(1u32)`
+   |     ^^^^^^^^^^^^ invalid cast
+   |
+help: try `char::from_u32` instead
+   |
+LL -     1u32 as char;
+LL +     char::from_u32(1u32);
+   |
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/error-codes/E0620.stderr b/tests/ui/error-codes/E0620.stderr
index 644ba813c96..b2efe02f3aa 100644
--- a/tests/ui/error-codes/E0620.stderr
+++ b/tests/ui/error-codes/E0620.stderr
@@ -2,9 +2,12 @@ error[E0620]: cast to unsized type: `&[usize; 2]` as `[usize]`
   --> $DIR/E0620.rs:2:16
    |
 LL |     let _foo = &[1_usize, 2] as [usize];
-   |                ^^^^^^^^^^^^^^^^^-------
-   |                                 |
-   |                                 help: try casting to a reference instead: `&[usize]`
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: try casting to a reference instead
+   |
+LL |     let _foo = &[1_usize, 2] as &[usize];
+   |                                 +
 
 error: aborting due to 1 previous error