This is the initial version of a buildable SIL definition in libswift.
It defines an initial set of SIL classes, like Function, BasicBlock, Instruction, Argument, and a few instruction classes.
The interface between C++ and SIL is a bridging layer, implemented in C.
It contains all the required bridging data structures used to access various SIL data structures.
* 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.
