cmState: Remove call stack parent tracking.

This was added at a time when I thought the cmState would report errors,
but that is not the direction it is going.  The ordering of the stack of
scopes is canonical and clients are required to maintain it.

Source/cmState.cxx

@@ -20,7 +20,6 @@
 
 struct cmState::SnapshotDataType
 {
-  cmState::PositionType CallStackParent;
   cmState::PositionType DirectoryParent;
   cmState::SnapshotType SnapshotType;
   cmLinkedTree<std::string>::iterator ExecutionListFile;
@@ -732,7 +731,6 @@ cmState::CreateBuildsystemDirectorySnapshot(Snapshot originSnapshot,
 {
   assert(originSnapshot.IsValid());
   PositionType pos = this->SnapshotData.Extend(originSnapshot.Position);
-  pos->CallStackParent = originSnapshot.Position;
   pos->EntryPointLine = entryPointLine;
   pos->EntryPointCommand = entryPointCommand;
   pos->DirectoryParent = originSnapshot.Position;
@@ -754,7 +752,6 @@ cmState::CreateFunctionCallSnapshot(cmState::Snapshot originSnapshot,
 {
   PositionType pos = this->SnapshotData.Extend(originSnapshot.Position,
                                                *originSnapshot.Position);
-  pos->CallStackParent = originSnapshot.Position;
   pos->EntryPointLine = entryPointLine;
   pos->EntryPointCommand = entryPointCommand;
   pos->SnapshotType = FunctionCallType;
@@ -772,7 +769,6 @@ cmState::CreateMacroCallSnapshot(cmState::Snapshot originSnapshot,
 {
   PositionType pos = this->SnapshotData.Extend(originSnapshot.Position,
                                                *originSnapshot.Position);
-  pos->CallStackParent = originSnapshot.Position;
   pos->EntryPointLine = entryPointLine;
   pos->EntryPointCommand = entryPointCommand;
   pos->SnapshotType = MacroCallType;
@@ -789,7 +785,6 @@ cmState::CreateCallStackSnapshot(cmState::Snapshot originSnapshot,
 {
   PositionType pos = this->SnapshotData.Extend(originSnapshot.Position,
                                                *originSnapshot.Position);
-  pos->CallStackParent = originSnapshot.Position;
   pos->EntryPointLine = entryPointLine;
   pos->EntryPointCommand = entryPointCommand;
   pos->SnapshotType = CallStackType;
@@ -806,7 +801,6 @@ cmState::CreateInlineListFileSnapshot(cmState::Snapshot originSnapshot,
 {
   PositionType pos = this->SnapshotData.Extend(originSnapshot.Position,
                                                *originSnapshot.Position);
-  pos->CallStackParent = originSnapshot.Position;
   pos->EntryPointLine = entryPointLine;
   pos->EntryPointCommand = entryPointCommand;
   pos->SnapshotType = InlineListFileType;
@@ -827,11 +821,7 @@ cmState::Snapshot cmState::Pop(cmState::Snapshot originSnapshot)
   prevPos->CompileOptionsPosition =
       prevPos->BuildSystemDirectory->CompileOptions.size();
 
-  if (prevPos == this->SnapshotData.Root())
-    {
-    return Snapshot(this, prevPos);
-    }
-  return Snapshot(this, originSnapshot.Position->CallStackParent);
+  return Snapshot(this, prevPos);
 }
 
 cmState::Snapshot::Snapshot(cmState* state)