swift234

CMake build: eliminate special cases and restore the creation of swift.swift

The previous commit broke generation of swift.swift, and that needed
to be fixed.  But swift.swift was created by concatenating all
sources, while other .swift files generated in the lib/swift/
directory were created by copying the single source file under
construction with a special-case "install" command.  Now we always
concatenate .swift sources into a .swift module source file in the
lib/swift/ directory.

Big thanks to Joe G. for his patient assistance

Swift SVN r4740