Fixed an infinite recursion bug that could happen when using python operating...

Fixed an infinite recursion bug that could happen when using python operating system plug-ins where we would ask the operating system plug-in to update its threads and this would cause the plugin to run an expression which would eventually run IRForTarget::CreateResultVariable() which would try to get the selected thread and cause re-entrant bug.

<rdar://problem/19924734>

llvm-svn: 230711