Change the Mac OS X SystemRuntime plugin from using the placeholder
libldi library to collect extended backtrace information; switch
to the libBacktraceRecording library and its APIs.  Complete the
work of adding QueueItems to Queues and allow for the QueueItems
to be interrogated about their extended backtraces in turn.

There's still cleanup and documentation to do on this code but the
code is functional and I it's a good time to get the work-in-progress
checked in.
<rdar://problem/15314027>