remove the "expand" diamond for variables where expanding fails

[llunak]

Summary

If the variables view shows a variable that is a struct that has MightHaveChildren(), the expand diamond is shown, but if trying to expand it and it's not possible (e.g. incomplete type), remove the expand diamond to visualize that it can't be in fact expanded. Otherwise it feels kinda weird that a tree item cannot be expanded even though it "should".

I guess the MightHaveChildren() checking means that GetChildren() may be expensive, so also do not call it for rows that are not expanded.

lldb/source/Core/IOHandlerCursesGUI.cpp

void Unexpand() {
     calculated_children = false;
     children.clear();
   }
 
   void DrawTree(Window &window) {
     if (parent)
       parent->DrawTreeForChild(window, this, 0);
 
-    if (might_have_children) {
+    if (might_have_children &&
+        (!calculated_children || !GetChildren().empty())) {
       // It we can get UTF8 characters to work we should try to use the
       // "symbol" UTF8 string below
       //            const char *symbol = "";
       //            if (row.expanded)
       //                symbol = "\xe2\x96\xbd ";
       //            else
       //                symbol = "\xe2\x96\xb7 ";
       //            window.PutCString (symbol);
for (auto &row : rows) {
           row.y = 0;
         }
       } else {
         row.x = 0;
         row.y = 0;
         ++m_num_rows;
       }
 
+      if (row.expanded) {
         auto &children = row.GetChildren();
-      if (row.expanded && !children.empty()) {
+        if (!children.empty()) {
           DisplayRows(window, children, options);
         }
       }
     }
+  }
 
   int CalculateTotalNumberRows(std::vector<Row> &rows) {
     int row_count = 0;
     for (auto &row : rows) {
       ++row_count;
       if (row.expanded)
         row_count += CalculateTotalNumberRows(row.GetChildren());
     }
     return row_count;
   }
 
   static Row *GetRowForRowIndexImpl(std::vector<Row> &rows, size_t &row_index) {
     for (auto &row : rows) {
       if (row_index == 0)
         return &row;
       else {
         --row_index;
+        if (row.expanded) {
           auto &children = row.GetChildren();
-        if (row.expanded && !children.empty()) {
+          if (!children.empty()) {
             Row *result = GetRowForRowIndexImpl(children, row_index);
             if (result)
               return result;
           }
         }
       }
+    }
     return nullptr;
   }
 
   Row *GetRowForRowIndex(size_t row_index) {
     return GetRowForRowIndexImpl(m_rows, row_index);
   }
 
   int NumVisibleRows() const { return m_max_y - m_min_y; }