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GH-42087: [Swift] refactored to remove build warnings (#42088)

### Rationale for this change
Building warnings were being generated from the swift code.

### What changes are included in this PR?
Refactored the code in ArrowCExporter and CodableTests to remove the build warnings.  

### Are these changes tested?
Yes, also ran CDataWGo tests locally to test CData interface changes.

* GitHub Issue: #42087

Authored-by: Alva Bandy <abandy@live.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>