NSSet: add missing convenience initializer

[egorzhdan]

This initializer exists in the macOS version of Foundation, but isn't present on Linux/Windows.
While it doesn't add any new functionality, it simplifies porting existing code to those platforms.

[MaxDesiatov]

@swift-ci please test

[compnerd]

I see https://developer.apple.com/documentation/foundation/nsset/1574825-setwithobjects?language=objc but it doesn't seem to have the corresponding interface in the Swift view. The change itself LGTM, assuming @millenomi doesn't know of a reason that the interface is missing. However, if there is a reason, it certainly should be indicated with a comment.

[egorzhdan]

It exists for Swift as well: https://developer.apple.com/documentation/foundation/nsset/1407975-init

I assume Swift isn't shown in the "Language" popup on https://developer.apple.com/documentation/foundation/nsset/1574825-setwithobjects?language=objc because this is a static method in ObjC but an initializer in Swift.

[compnerd]

Lets wait for tomorrow just to allow @millenomi a chance to look at it as well, but with that last bit, I think that this is perfectly fine.

[millenomi]

@swift-ci please test and merge