Fix a race condition in Target::Launch

When no hijack listener is set up, the global event listener will
try to pull events off the queue, racing with the event thread.
By always forcing a hijack listener, even when one was not given,
we guarantee that the listener always gets all events.

This was causing problems in synchronous mode with the process
stop event sometimes never being picked up and causing the debugger
to hang while processing a .lldbinit file.

Reviewed by: Jim Ingham
Differential Revision: http://reviews.llvm.org/D8562

llvm-svn: 233315