[CMake] Prevent rebuilds of pure swift libraries and executables
`.swiftmodule` is listed as an output of pure swift libraries, but it
isn't updated if there's been no changes (just like the library and
its object files). Add it to the touch hack as well.
Swift *executables* also list `.swiftmodule` as an output, even though
one isn't generated at all. Add a new POST_BUILD command to touch it
regardless.
