No source fixes

[Muscraft]

This PR started as a fix for a rendering bug that got noticed in #143661, 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

[rustbot]

r? @SparrowLii

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[rustbot]

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.