Thread hardening part 3. Now lldb_private::Thread objects have std::weak_ptr
objects for the backlink to the lldb_private::Process. The issues we were
running into before was someone was holding onto a shared pointer to a 
lldb_private::Thread for too long, and the lldb_private::Process parent object
would get destroyed and the lldb_private::Thread had a "Process &m_process"
member which would just treat whatever memory that used to be a Process as a
valid Process. This was mostly happening for lldb_private::StackFrame objects
that had a member like "Thread &m_thread". So this completes the internal
strong/weak changes.

Documented the ExecutionContext and ExecutionContextRef classes so that our
LLDB developers can understand when and where to use ExecutionContext and 
ExecutionContextRef objects.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@151009 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/source/Plugins/Process/Utility/UnwindLLDB.cpp b/source/Plugins/Process/Utility/UnwindLLDB.cpp
index bf71b00..1ba3f0f 100644
--- a/source/Plugins/Process/Utility/UnwindLLDB.cpp
+++ b/source/Plugins/Process/Utility/UnwindLLDB.cpp
@@ -42,7 +42,8 @@
         if (!AddFirstFrame ())
             return 0;
 
-        ABI *abi = m_thread.GetProcess().GetABI().get();
+        ProcessSP process_sp (m_thread.GetProcess());
+        ABI *abi = process_sp ? process_sp->GetABI().get() : NULL;
 
         while (AddOneMoreFrame (abi))
         {
@@ -186,7 +187,8 @@
             return false;
     }
 
-    ABI *abi = m_thread.GetProcess().GetABI().get();
+    ProcessSP process_sp (m_thread.GetProcess());
+    ABI *abi = process_sp ? process_sp->GetABI().get() : NULL;
 
     while (idx >= m_frames.size() && AddOneMoreFrame (abi))
         ;
@@ -217,7 +219,8 @@
             return reg_ctx_sp;
     }
 
-    ABI *abi = m_thread.GetProcess().GetABI().get();
+    ProcessSP process_sp (m_thread.GetProcess());
+    ABI *abi = process_sp ? process_sp->GetABI().get() : NULL;
 
     while (idx >= m_frames.size())
     {