This commit does two things.  One, it converts the return value of the QueueThreadPlanXXX 
plan providers from a "ThreadPlan *" to a "lldb::ThreadPlanSP".  That was needed to fix
a bug where the ThreadPlanStepInRange wasn't checking with its sub-plans to make sure they
succeed before trying to proceed further.  If the sub-plan failed and as a result didn't make
any progress, you could end up retrying the same failing algorithm in an infinite loop.

<rdar://problem/14043602>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@186618 91177308-0d34-0410-b5e6-96231b3b80d8