[CMake] Manually Set swift-version
[CMake] Manually Set swift-version

CMAKE_Swift_LANGUAGE_VERSION isn't supported until CMake 3.16 which
isn't generally released yet. For anything less, pass in -swift-version
4.2 with add_compile_options.

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