[Fix] ZXing broken subspec inheritance. Caused by #9626 and CocoaPods/CocoaPods#1523, fixed by moving to normal multi-platform support for iOS dependencies. Add a dummy 'ios' subspec for backward compatibility with old Podfiles.
[Fix] Remove empty subspec in ZXing for backwards compatibility, as it does not pass lint.
Merge pull request #10075 from Pretz/master

[Fix] ZXing broken subspec inheritance.