swift234

Turn on module building by default in CMake.

Note that if you already have a CMake build, you will need to
 (1) re-run CMake to turn on the SWIFT_USE_LIBRARY_MODULES option, and
 (2) erase build/lib/**/swift/*.swift
     (this will be lib/swift for a command-line build and lib/*/swift for
      an Xcode-based build)

Swift SVN r6532