Rename @exported to @_exported
Rename @exported to @_exported

cf. https://github.com/apple/swift/commit/6e1bf0d10d0c2133b39edaff6f234e353dc58b3e

IngmarStein/CommandLine,jatoben/CommandLine,deszip/CommandLine,sebcode/CommandLine,deszip/CommandLine,IngmarStein/CommandLine,deszip/CommandLine,sebcode/CommandLine,jatoben/CommandLine