Merge pull request #13314 from jordan-woyak/input-expressions-assignment-op-fix

InputCommon: Fix input expression assignment operator behavior.

Source/Core/InputCommon/ControlReference/ExpressionParser.h

@@ -170,10 +170,13 @@ class Expression
 {
 public:
   virtual ~Expression() = default;
-  virtual ControlState GetValue() const = 0;
+  virtual ControlState GetValue() = 0;
   virtual void SetValue(ControlState state) = 0;
   virtual int CountNumControls() const = 0;
   virtual void UpdateReferences(ControlEnvironment& finder) = 0;
+
+  // Perform any side effects and return Expression to be SetValue'd.
+  virtual Expression* GetLValue();
 };
 
 class ParseResult