On long spans, trim the middle of them to make them fit in the terminal width

When encountering a single line span that is wider than the terminal, we keep context at the start and end of the span but otherwise remove the code from the middle. This is somewhat independent from whether the left and right margins of the output have been trimmed as well.

```
error[E0308]: mismatched types
  --> $DIR/long-span.rs:6:15
   |
LL | ... = [0, 0, 0, 0, ..., 0, 0];
   |       ^^^^^^^^^^^^^...^^^^^^^ expected `u8`, found `[{integer}; 1681]`
```

Address part of #137680 (missing handling of the long suggestion). Fix #125581.

compiler/rustc_errors/src/styled_buffer.rs

@@ -89,6 +89,16 @@ impl StyledBuffer {
         }
     }
 
+    pub(crate) fn replace(&mut self, line: usize, start: usize, end: usize, string: &str) {
+        if start == end {
+            return;
+        }
+        let _ = self.lines[line].drain(start..(end - string.chars().count()));
+        for (i, c) in string.chars().enumerate() {
+            self.lines[line][start + i] = StyledChar::new(c, Style::LineNumber);
+        }
+    }
+
     /// For given `line` inserts `string` with `style` before old content of that line,
     /// adding lines if needed
     pub(crate) fn prepend(&mut self, line: usize, string: &str, style: Style) {