When displaying multispans, ignore empty lines adjacent to ...

```
error[E0308]: `match` arms have incompatible types
   --> tests/ui/codemap_tests/huge_multispan_highlight.rs:98:18
    |
6   |       let _ = match true {
    |               ---------- `match` arms have incompatible types
7   |           true => (
    |  _________________-
8   | |             // last line shown in multispan header
...   |
96  | |
97  | |         ),
    | |_________- this is found to be of type `()`
98  |           false => "
    |  __________________^
...   |
119 | |
120 | |         ",
    | |_________^ expected `()`, found `&str`

error[E0308]: `match` arms have incompatible types
   --> tests/ui/codemap_tests/huge_multispan_highlight.rs:215:18
    |
122 |       let _ = match true {
    |               ---------- `match` arms have incompatible types
123 |           true => (
    |  _________________-
124 | |
125 | |         1 // last line shown in multispan header
...   |
213 | |
214 | |         ),
    | |_________- this is found to be of type `{integer}`
215 |           false => "
    |  __________________^
216 | |
217 | |
218 | |         1 last line shown in multispan
...   |
237 | |
238 | |         ",
    | |_________^ expected integer, found `&str`
```

tests/ui/async_yields_async.stderr

@@ -81,8 +81,7 @@ LL |       let _m = async || {
    |  _______________________-
 LL | |         println!("I'm bored");
 LL | |         // Some more stuff
-LL | |
-LL | |         // Finally something to await
+...  |
 LL | |         CustomFutureType
    | |         ^^^^^^^^^^^^^^^^
    | |         |

tests/ui/empty_line_after_outer_attribute.stderr

@@ -22,8 +22,7 @@ error: found an empty line after an outer attribute. Perhaps you forgot to add a
   --> tests/ui/empty_line_after_outer_attribute.rs:28:1
    |
 LL | / #[crate_type = "lib"]
-LL | |
-LL | |
+...  |
 LL | | fn with_two_newlines() { assert!(true) }
    | |_