[SYCL] Refactor sycl-post-link saveModule to improve code structure (#21674)
Extract prepareModuleBeforeSave() helper and change saveModule() to
return Error for better error handling. Move output file index handling
to call sites for more flexible function design.
Output filenames changed from prefix_esimd_N.ext to prefix_N.esimd.ext
as a consequence of the refactoring.
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Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>