Add logging for the SB API which creates extended
threads.

Take a stab at fixing the too-soon freeing of the extended
backtrace thread list in Process.
<rdar://problem/15496603> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@195104 91177308-0d34-0410-b5e6-96231b3b80d8