[lldb][DataFormatter][NFC] Use GetFirstValueOfLibCXXCompressedPair in std::map formatter

This avoids duplicating the logic to get the first
element of a libc++ `__compressed_pair`. This will
be useful in upcoming refactorings of this formatter.

Drive-by changes:
* Renamed `m_item` to `size_node` for readability;
  `m_item` suggests it's a member variable, which it
  is not.

Author: Michael137

lldb/source/Plugins/Language/CPlusPlus/LibCxxMap.cpp

@@ -213,30 +213,20 @@ size_t lldb_private::formatters::LibcxxStdMapSyntheticFrontEnd::
     CalculateNumChildren() {
   if (m_count != UINT32_MAX)
     return m_count;
+
   if (m_tree == nullptr)
     return 0;
-  ValueObjectSP m_item(m_tree->GetChildMemberWithName("__pair3_"));
-  if (!m_item)
+
+  ValueObjectSP size_node(m_tree->GetChildMemberWithName("__pair3_"));
+  if (!size_node)
     return 0;
 
-  switch (m_item->GetCompilerType().GetNumDirectBaseClasses()) {
-  case 1:
-    // Assume a pre llvm r300140 __compressed_pair implementation:
-    m_item = m_item->GetChildMemberWithName("__first_");
-    break;
-  case 2: {
-    // Assume a post llvm r300140 __compressed_pair implementation:
-    ValueObjectSP first_elem_parent = m_item->GetChildAtIndex(0);
-    m_item = first_elem_parent->GetChildMemberWithName("__value_");
-    break;
-  }
-  default:
-    return false;
-  }
+  size_node = GetFirstValueOfLibCXXCompressedPair(*size_node);
 
-  if (!m_item)
+  if (!size_node)
     return 0;
-  m_count = m_item->GetValueAsUnsigned(0);
+
+  m_count = size_node->GetValueAsUnsigned(0);
   return m_count;
 }