swift234

Switch to the new swift/swiftc commmand line interface

This makes the command line interface to 'swift' the same as what was
previously in 'swifti', and removes the staging symlink.

For posterity, the command line behaviour for 'swift' is now:
* `swift` -> start the repl
* `swift script.swift` -> run script.swift (the old -i mode)
* Any arguments after the input file are forwarded to the script as
  Process.arguments
* A shebang line is something like #!/usr/bin/xcrun swift

The batch compiler 'swiftc' behaves much like the old 'swift'
executable, but without the interactive bits now in 'swifti'.

<rdar://problem/17710788>

Swift SVN r20540