Add new Queue, QueueItem, Queuelist, SBQueue, SBQueueItem classes to represent
libdispatch aka Grand Central Dispatch (GCD) queues.  Still fleshing out the
documentation and testing of these but the overall API is settling down so it's
a good time to check it in.
<rdar://problem/15600370> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@197190 91177308-0d34-0410-b5e6-96231b3b80d8