Add the ability to attach notes to Diagnostic/InFlightDiagnostic.

    Notes are a way to add additional context to a diagnostic, but don't really make sense as standalone diagnostics. Moving forward, notes will no longer be able to be constructed directly and must be attached to a parent Diagnostic.

    Notes can be attached via `attachNote`:

      auto diag = ...;
      diag.attachNote() << "This is a note";

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PiperOrigin-RevId: 246545971