So I finally updated to XCode 9.4 today, then tried to build an app and submit it to the store. This was a bug fix, so there shouldn't have been any changes in the structure, no new frameworks, etc. I've been submitting this app for several years with no problems.
I get rejected with --
Invalid Swift Support - The files libswiftDarwin.dylib, libswiftMetal.dylib, libswiftCoreAudio.dylib, libswiftsimd.dylib, libswiftQuartzCore.dylib, libswiftos.dylib, libswiftMapKit.dylib, libswiftObjectiveC.dylib, libswiftDispatch.dylib, libswiftCoreLocation.dylib, libswiftCoreGraphics.dylib, libswiftCoreFoundation.dylib, libswiftUIKit.dylib, libswiftCoreMedia.dylib, libswiftAVFoundation.dylib, libswiftCore.dylib, libswiftGLKit.dylib, libswiftFoundation.dylib, libswiftModelIO.dylib, libswiftXCTest.dylib, libswiftWatchKit.dylib, libswiftCoreImage.dylib aren’t at the expected location /Payload/[appname].app/Frameworks. Move the file to the expected location, rebuild your app using the current public (GM) version of Xcode, and resubmit it.
Why? I don't see any loose .dylib files, which is what the little documentation I could find says is the likely culprit. Those libraries are in the /Frameworks directory, along with my 3rd Party frameworks. The archive from the organizer looks to be identical to the one I sucessfully submitted last month.
This is frustrating, because I don't think I changed anything, and can't see anything different from previous submissions.
Turns out the problem appears to have been the libswiftXCTest.sylib file in the /SwiftSupport/iphoneos directory, which is at the same level as the /Products directory (Note, I don't see /Payload anywhere). Not sure when that appeared in there, as I've had unit tests running for quite a while, but it looks like that was my problem. Just a really uniformative error message, one that didn't tell me the real problem.
Got rid the unit test scheme in the Scheme Editor's Build/Target pane, and was able to sucessfully submit. (Will figure out what's the right way to re-enable my unit tests after I get submitted and sucessfully reviewed...)
Posting this to help anyone else who might be having a similar problem.