swift

9886b635 - [AutoDiff] Rename 'AutoDiffIndexSubset' to 'IndexSubset'. (#27615)

[AutoDiff] Rename 'AutoDiffIndexSubset' to 'IndexSubset'. (#27615) Per @DougGregor's review comment (https://github.com/apple/swift/pull/27555#discussion_r333757205), `AutoDiffIndexSubset` data structure could have a more general name. We rename it to `IndexSubset` and move it to its own file.