* add the (still empty) libswift package
* add build support for libswift in CMake
* add libswift to swift-frontend and sil-opt
The build can be controlled with the LIBSWIFT_BUILD_MODE cmake variable: by default it’s “DISABLE”, which means that libswift is not built. If it’s “HOSTTOOLS”, libswift is built with a pre-installed toolchain on the host system.
