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.

llvm-svn: 151009

lldb/source/Target/ThreadPlanStepInRange.cpp

@@ -108,7 +108,7 @@ ThreadPlanStepInRange::ShouldStop (Event *event_ptr)
     {
         StreamString s;
         s.Address (m_thread.GetRegisterContext()->GetPC(), 
-                   m_thread.GetProcess().GetTarget().GetArchitecture().GetAddressByteSize());
+                   m_thread.CalculateTarget()->GetArchitecture().GetAddressByteSize());
         log->Printf("ThreadPlanStepInRange reached %s.", s.GetData());
     }