[bootstrap] Generate xcconfig file before generating the project
[bootstrap] Generate xcconfig file before generating the project

Generating xcconfig file post project generation doesn't work now
because we started checking if the file exists.

sschiau/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,vmanot/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,JGiola/swift-package-manager,sschiau/swift-package-manager