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>;
   |                    ++++        +
```

Part of rust-lang#141973.

Rollup of 8 pull requests

Successful merges:

 - #137992 (Stabilise `os_string_pathbuf_leak`)
 - #141558 (Limit the size of cgu names when using the `-Zhuman-readable-cgu-name…)
 - #141797 (compiler: set Apple frame pointers by architecture)
 - #141857 (coretests: move float tests from num to floats module and use a more flexible macro to generate them)
 - #142045 (Make obligation cause code suggestions verbose)
 - #142076 (Check documentation of bootstrap in PR CI)
 - #142110 (Add solaris targets to build-manifest)
 - #142131 (Make cast suggestions verbose)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of #142131 - estebank:cast-sugg, r=Urgau

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>;
   |                    ++++        +
```

Part of #141973.