Move the ivars / logic of SBQueue into a QueueImpl class and 
change SBQueue to have a shared pointer to its corresponding
QueueImpl object for binary compatibility.
<rdar://problem/15657926> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@197300 91177308-0d34-0410-b5e6-96231b3b80d8