Access swift toolchain's frontend using Xcode

I want to emit certain things available in swift -frontend mode. .swiftinterface to be concrete. The problem is that I want to do it for the pretty complex framework that itself a CocoaPod and depends on other Pods.

To create a buildable copy of the codebase I use cocoapods-generate to generate an Xcodeproj for a particular framework. However when I try to add flags to OTHER_SWIFT_FLAGS they are being added to swiftc callings, not swift.

So the question is: given that I have a buildable Xcode scheme, how do I run any swift -frontend things on it?