swift

483a3787 - [AutoDiff] NFC: Move some 'IndexSubset' method impls to their own file. (#27627)

[AutoDiff] NFC: Move some 'IndexSubset' method impls to their own file. (#27627) - Move some 'IndexSubset' method implemenetations from AutoDiff.cpp to a new file called IndexSubset.cpp. - Fix some indentation errors caused by the earlier renaming of `AutoDiffIndexSubset`.