[lldb/Utility] Add opt-in shadow mode to event listeners

This patch augments lldb's event listeners with a new shadow mode.

As the name suggests, this mode allows events to be copied to an
additional listener to perform event monitoring, without interferring
with the event life cycle.

One of our use case for this, is to be able to listen to public process
events while making sure the events will still be delivered to the
default process listener (the debugger listener in most cases).

Differential Revision: https://reviews.llvm.org/D148397



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>