No source fixes
This PR started as a fix for a rendering bug that [got noticed in #143661](https://github.com/rust-lang/rust/pull/143661#discussion_r2199109530), but turned into a fix for any rendering bugs related to files with no source.
- Don't add an end column separator after a file with no source
- Add column separator before secondary messages with no source
- Render continuation between no source labels
Before
```
error[E0423]: expected function, tuple struct or tuple variant, found struct `std::collections::HashMap`
╭▸ $DIR/multi-suggestion.rs:17:13
│
LL │ let _ = std::collections::HashMap();
│ ━━━━━━━━━━━━━━━━━━━━━━━━━━━
╭▸ $SRC_DIR/std/src/collections/hash/map.rs:LL:COL
│
╰ note: `std::collections::HashMap` defined here
╰╴
note: constructor is not visible here due to private fields
╭▸ $SRC_DIR/alloc/src/boxed.rs:LL:COL
│
╰ note: private field
│
╰ note: private field
```
After
```
error[E0423]: expected function, tuple struct or tuple variant, found struct `std::collections::HashMap`
╭▸ $DIR/multi-suggestion.rs:17:13
│
LL │ let _ = std::collections::HashMap();
│ ━━━━━━━━━━━━━━━━━━━━━━━━━━━
╰╴
╭▸ $SRC_DIR/std/src/collections/hash/map.rs:LL:COL
│
╰ note: `std::collections::HashMap` defined here
note: constructor is not visible here due to private fields
╭▸ $SRC_DIR/alloc/src/boxed.rs:LL:COL
│
├ note: private field
│
╰ note: private field
```
Note: This PR also makes it so `rustc` and `annotate-snippets` match in these cases
2fae59ac96