Add support for Swift to CMake
CMake is currently unable to generate rules for building swift libraries/executables. Trying to do this out-of-band does not scale very well and has become problematic. Adding language support for Swift has found another issue with CMake - it does not currently provide a way to enumerate all the sources for a target. Swift requires the ability to know all the sources in order to perform type checking across the source files. In order to follow the compile+link model, we need the ability to have the generate enumerate all the sources. This need was discussed on cmake-developers (https://cmake.org/pipermail/cmake-developers/2019-January/030970.html) and did not seem to be something that was too objectionable. An initial attempt was made at !2797 (merged) to address this need.
@brad.king - what would you suggest for the placeholder to get access to the [remaining] source files in a target compilation?