[SwiftDiagnostics] Render attached notes in GroupedDiagnostics

[ihormalovanyi]

Summary

GroupedDiagnostics only handed the top-level diagnostics to DiagnosticsFormatter, so any notes attached to a Diagnostic — which have their own location and message — were never rendered. This fixes #2166 by expanding each diagnostic's notes into .note-severity diagnostics before formatting, so they are laid out at their locations the same way top-level diagnostics are.

Implementation

• Added a small NoteAsDiagnosticMessage adapter that presents a NoteMessage as a .note-severity DiagnosticMessage, plus a Note.asDiagnostic helper. Both are file-private; there is no public API change.
• annotateSource now flattens [diagnostic] + diagnostic.notes before passing the list to the formatter. Primary-diagnostic selection is unchanged, so the header still reports the error/warning rather than a note.

Testing

• New testAttachedNotesAreRendered, covering a diagnostic with one attached note.
• testGroupingForMacroExpansion's expected output is updated: the parser's real "to match this opening '('" note attached to the "expected ')'" error is now rendered (it was previously dropped), which doubles as evidence of the fix.

Out of scope (noted in #2166)

• Fix-Its are still not rendered by GroupedDiagnostics.
• Notes that point into a different source buffer than their diagnostic are flattened as-is; refining cross-buffer note placement could be done separately.