Issues building latest master for Xcode development

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I hope this is an appropriate place to ask for help with this. If not, please let me know if there is a better place! I was looking to dive into some Swift compiler development and I've immediately hit a brick wall with the latest build.

I've pulled down master (I'm currently on commit 6f94f6c0ef4), followed the directions from the README.md:

  1. Used homebrew to install dependencies: cmake and ninja
  2. Downloaded/installed Xcode 10 beta and made sure it is selected with xcode-select. Output of xcodebuild -version is Xcode 10.0 Build version 10L176w
  3. Created a swift-source directory to place the code in
  4. Cloned Swift and ran the script to pull down other projects git clone git@github.com:apple/swift.git & ./swift/utils/update-checkout --clone-with-ssh.
  5. Ran utils/build-script --debug --xcode to make a debug build I can run with Xcode.

Whenever running step 4 above, I end up getting:

The following build commands failed:
	CompileC /Users/mcmcguir/github/swift-source/build/Xcode-DebugAssert/swift-macosx-x86_64/stdlib/public/stubs/Swift.build/Debug/swiftStdlibStubs-macosx-x86_64.build/Objects-normal/x86_64/SwiftNativeNSXXXBase.o /Users/mcmcguir/github/swift-source/build/Xcode-DebugAssert/swift-macosx-x86_64/stdlib/public/stubs/8/SwiftNativeNSXXXBase.mm normal x86_64 objective-c++ com.apple.compilers.llvm.clang.1_0.compiler
(1 failure)
utils/build-script: fatal error: command terminated with a non-zero exit status 65, aborting

This is the error I'm seeing:

error: Build input file cannot be found: '/Users/mcmcguir/github/swift-source/build/Xcode-DebugAssert/swift-macosx-x86_64/stdlib/public/stubs/8/SwiftNativeNSXXXBase.mm'

If I copy the entire command and run it, it seems to execute just fine. I see the file present in that location. If I just run utils/build-script --debug it seems to finish ok.

Any ideas?

Thanks!