CheatSearchWidget: Rename member variable

Rename m_session to m_last_value_session to better reflect its meaning
and distinguish it from m_current_value_session which will be introduced
in another commit.

Source/Core/DolphinQt/CheatSearchWidget.h

@@ -77,7 +77,9 @@ private:
 
   Core::System& m_system;
 
-  std::unique_ptr<Cheats::CheatSearchSessionBase> m_session;
+  // Stores the values used by the "last value" search filter and shown in the Last Value column.
+  // Updated only after clicking the "Search and Filter" button or resetting the table.
+  std::unique_ptr<Cheats::CheatSearchSessionBase> m_last_value_session;
 
   // storage for the 'Current Value' column's data
   std::unordered_map<u32, std::string> m_address_table_current_values;