When a client asks for a queue pending item's extended backtrace,
hold a strong pointer to that extended backtrace thread in the Process
just like we do for asking a thread's extended backtrace.
Also, give extended backtrace threads an invalid ThreadIndexID number.
We'll still give them valid thread_id's.  Clients who want to know the
original thread's IndexID can call GetExtendedBacktraceOriginatingIndexID().
<rdar://problem/16126034> 

llvm-svn: 203088
diff --git a/lldb/source/Plugins/Process/Utility/HistoryUnwind.cpp b/lldb/source/Plugins/Process/Utility/HistoryUnwind.cpp
index 86665fd..4659e48 100644
--- a/lldb/source/Plugins/Process/Utility/HistoryUnwind.cpp
+++ b/lldb/source/Plugins/Process/Utility/HistoryUnwind.cpp
@@ -20,6 +20,8 @@
 using namespace lldb;
 using namespace lldb_private;
 
+// Constructor 
+
 HistoryUnwind::HistoryUnwind (Thread &thread,
                               std::vector<lldb::addr_t> pcs,
                               uint32_t stop_id,
@@ -31,6 +33,8 @@
 {
 }
 
+// Destructor
+
 HistoryUnwind::~HistoryUnwind ()
 {
 }