Hey!
If you use the 4.17.1 from the launcher and then install the Xcode beta app, use the terminal command to reroute where UE looks for Xcode, you should be able to get iOS 11 and ue4 playing nicely.
Granted, now that the official ios / sdk 11 is out, I don’t think “beta” xcode is around anymore so you’ll have to use the official Xcode 9 version for ios11. I’m not sure Epic has updated to support the official release just yet, unless you’re willing to jump to 4.18 preview 1, which may break other features.
Ultimately, I was having this issue when UE4 was pointing to a beta version of Xcode that itself had metal bugs. Those have since been patched, and as long as you have the right version of Xcode matched with unreal, it should let you compile shaders without issue, albeit some time lag.