Teach diagnostics to have correctly padded lists

Make the suggestion list have a correct padding:

```
error[E0308]: mismatched types
 --> file.rs:3:20
  |
3 |     let x: usize = "";
  |                    ^^ expected usize, found reference
  |
  = note: expected type `usize`
  = note:    found type `&'static str`
  = help: here are some functions which might fulfill your needs:
          - .len()
          - .foo()
          - .bar()
```

src/test/ui/resolve/token-error-correct-3.stderr

@@ -37,9 +37,9 @@ error[E0308]: mismatched types
    = note: expected type `()`
    = note:    found type `std::result::Result<bool, std::io::Error>`
    = help: here are some functions which might fulfill your needs:
- - .unwrap()
- - .unwrap_err()
- - .unwrap_or_default()
+           - .unwrap()
+           - .unwrap_err()
+           - .unwrap_or_default()
 
 error: aborting due to previous error
 

src/test/ui/span/coerce-suggestions.stderr

@@ -7,8 +7,8 @@ error[E0308]: mismatched types
    = note: expected type `usize`
    = note:    found type `std::string::String`
    = help: here are some functions which might fulfill your needs:
- - .capacity()
- - .len()
+           - .capacity()
+           - .len()
 
 error[E0308]: mismatched types
   --> $DIR/coerce-suggestions.rs:23:19
@@ -19,10 +19,10 @@ error[E0308]: mismatched types
    = note: expected type `&str`
    = note:    found type `std::string::String`
    = help: here are some functions which might fulfill your needs:
- - .as_str()
- - .trim()
- - .trim_left()
- - .trim_right()
+           - .as_str()
+           - .trim()
+           - .trim_left()
+           - .trim_right()
 
 error[E0308]: mismatched types
   --> $DIR/coerce-suggestions.rs:30:10