[bootstrap] Forward swift compiler path when building llbuild
[bootstrap] Forward swift compiler path when building llbuild

<rdar://problem/44583560> Swiftpm needs to forward --swiftc to llbuild

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