swift234

[StdlibUnittest] Install our own handler for uncaught ObjC exceptions.

We don't want to be at the whims of the system on what to do with an
uncaught exception; we need to make sure its message gets printed to
stderr so that the parent process can check it.

(There's a bit of trickery here to see if the class looks like an
NSException; otherwise we lose the name of the exception and just get
the reason.)