Rework how master plans declare themselves.  Also make "PlanIsBasePlan" not rely only on this being the bottom plan in the stack, but allow the plan to declare itself as such.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@154351 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/source/Target/ThreadPlan.cpp b/source/Target/ThreadPlan.cpp
index 8e9cb47..5f396d9 100644
--- a/source/Target/ThreadPlan.cpp
+++ b/source/Target/ThreadPlan.cpp
@@ -36,7 +36,8 @@
     m_plan_complete_mutex (Mutex::eMutexTypeRecursive),
     m_plan_complete (false),
     m_plan_private (false),
-    m_okay_to_discard (false)
+    m_okay_to_discard (false),
+    m_is_master_plan (false)
 {
     SetID (GetNextID());
 }