Improve error message when trying to modify constant variables

libsolidity/ast/ASTAnnotations.h

@@ -154,6 +154,8 @@ struct ExpressionAnnotation: ASTAnnotation
 {
 	/// Inferred type of the expression.
 	TypePointer type;
+	/// Whether the expression is a constant variable
+	bool isConstant = false;
 	/// Whether it is an LValue (i.e. something that can be assigned to).
 	bool isLValue = false;
 	/// Whether the expression is used in a context where the LValue is actually required.