[BuildPlan] Moved -Xcxx flags to the end of the chain of arguments (#1380)
[BuildPlan] Moved -Xcxx flags to the end of the chain of arguments (#1380)

* [BuildPlan] In reference to SR-6164, moved -Xcxx flags to the end of the chain of arguments.

    - https://bugs.swift.org/browse/SR-6164

* Commented the constraints for argument ordering to allow user overrides
of -Xswiftc, -Xlinker, -Xcc and -Xcxx.

- https://bugs.swift.org/browse/SR-6164

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