Swift/Ninja: Set copy-if-different on output-file-map.json

Avoid bumping the file's mtime on every configure when its content
hasn't changed.  Otherwise ninja sees it as modified and re-runs every
Swift compile edge that depends on it.

Observed in the Swift toolchain build (LLVM monorepo with swift as an
external project): a no-op reconfigure cascaded into hundreds of
unnecessary edges via per-target output-file-map.json mtime bumps.

Issue: #27748