This patch captures and serializes all output being written by the
command line driver, including the lldb prompt being output by
editline, the asynchronous process output & error messages, and
asynchronous messages written by target stop-hooks.

As part of this it introduces a new Stream class,
StreamAsynchronousIO.  A StreamAsynchronousIO object is created with a
broadcaster, who will eventually broadcast the stream's data for a
listener to handle, and an event type indicating what type of event
the broadcaster will broadcast.  When the Write method is called on a
StreamAsynchronousIO object, the data is appended to an internal
string.  When the Flush method is called on a StreamAsynchronousIO
object, it broadcasts it's data string and clears the string.

Anything in lldb-core that needs to generate asynchronous output for
the end-user should use the StreamAsynchronousIO objects.

I have also added a new notification type for InputReaders, to let
them know that a asynchronous output has been written. This is to
allow the input readers to, for example, refresh their prompts and
lines, if desired.  I added the case statements to all the input
readers to catch this notification, but I haven't added any code for
handling them yet (except to the IOChannel input reader).

llvm-svn: 130721