[PackageLoading] Allow using Foundation in PackageDescription
[PackageLoading] Allow using Foundation in PackageDescription

This will add the arguments that are required to import Foundation in
PackageDescription library when bootstrapping on the CI. It should be
now possible to use Codable for generating JSON in the runtime
libraries.

<rdar://problem/45975602> Allow importing Foundation in the PackageDescription library

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