XcodeProj: Compile error in release builds

It seems Swift 4.2 is failing to build xcodeproj in release mode for some reason. This can be seen by running:

swift build -c release

The following error occurs

PHI node operands are not the same type as the result!
  %159 = phi %Ts23_ContiguousArrayStorageCySSG* [ %336, %328 ], [ %143, %481 ], [ %143, %470 ], [ %143, %454 ], [ %143, %438 ], [ %143, %423 ], [ %143, %146 ]
<unknown>:0: error: fatal error encountered during compilation; please file a bug report with your project and the crash log
<unknown>:0: note: Broken function found, compilation aborted!
0  swift                    0x000000010e76664a PrintStackTraceSignalHandler(void*) + 42
1  swift                    0x000000010e765dfe SignalHandler(int) + 302
2  libsystem_platform.dylib 0x00007fff7908df5a _sigtramp + 26
3  libsystem_platform.dylib 0x000000011b583600 _sigtramp + 2723108544
4  libsystem_c.dylib        0x00007fff78e2b1ae abort + 127
5  swift                    0x000000010a9edaf7 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*)::$_1::__invoke(void*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, bool) + 519
6  swift                    0x000000010e71a278 llvm::report_fatal_error(llvm::Twine const&, bool) + 280
7  swift                    0x000000010e71a15b llvm::report_fatal_error(char const*, bool) + 43
8  swift                    0x000000010e6e17e6 (anonymous namespace)::VerifierLegacyPass::runOnFunction(llvm::Function&) + 54
9  swift                    0x000000010e68804e llvm::FPPassManager::runOnFunction(llvm::Function&) + 446
10 swift                    0x000000010e690363 llvm::legacy::FunctionPassManagerImpl::run(llvm::Function&) + 115
11 swift                    0x000000010e690258 llvm::legacy::FunctionPassManager::run(llvm::Function&) + 392
12 swift                    0x000000010abb4228 swift::performLLVM(swift::IRGenOptions&, swift::DiagnosticEngine*, llvm::sys::SmartMutex<false>*, llvm::GlobalVariable*, llvm::Module*, llvm::TargetMachine*, swift::version::Version const&, llvm::StringRef, swift::UnifiedStatsReporter*) + 4504
13 swift                    0x000000010abb8e0e ThreadEntryPoint(swift::irgen::IRGenerator*, llvm::sys::SmartMutex<false>*, int) + 206
14 swift                    0x000000010abb8e6f void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(swift::irgen::IRGenerator*, llvm::sys::SmartMutex<false>*, int), swift::irgen::IRGenerator*, llvm::sys::SmartMutex<false>*, int> >(void*) + 47
15 libsystem_pthread.dylib  0x00007fff79097661 _pthread_body + 340
16 libsystem_pthread.dylib  0x00007fff7909750d _pthread_body + 0
17 libsystem_pthread.dylib  0x00007fff79096bf9 thread_start + 13
Stack dump:
0.	Running pass 'Module Verifier' on function '@"$S9xcodeproj12PBXBuildFileC16plistKeyAndValue4proj9referenceAA15CommentedStringV3key_AA05PlistG0O5valuetAA7PBXProjC_SStKFTf4dxn_n"'
<unknown>:0: error: unable to execute command: Abort trap: 6
<unknown>:0: error: compile command failed due to signal 6 (use -v to see invocation)

This is a blocker for XcodeGen and there is an open issue here https://github.com/yonaskolb/XcodeGen/issues/391