swift234

stdlib: Add a 'replExit' hook.

In the repl, if a replExit function is defined, call it when the repl is exiting. In the stdlib, add an atREPLExit function that pushes replExit handlers onto a vector and a replExit function that invokes those handlers. In swiftAppKit, have REPLApplicationMain use atREPLExit to terminate the application when the REPL is exited.

Swift SVN r4132