Use const references in string_constraintt

Author: Joel Allred

src/solvers/refinement/string_constraint.cpp

@@ -28,10 +28,10 @@ static bool cannot_be_neg(const exprt &expr)
 }
 
 string_constraintt::string_constraintt(
-  symbol_exprt _univ_var,
-  exprt lower_bound,
-  exprt upper_bound,
-  exprt body)
+  const symbol_exprt &_univ_var,
+  const exprt &lower_bound,
+  const exprt &upper_bound,
+  const exprt &body)
   : univ_var(_univ_var),
     lower_bound(lower_bound),
     upper_bound(upper_bound),

src/solvers/refinement/string_constraint.h

@@ -68,10 +68,10 @@ class string_constraintt final
   string_constraintt() = delete;
 
   string_constraintt(
-    symbol_exprt _univ_var,
-    exprt lower_bound,
-    exprt upper_bound,
-    exprt body);
+    const symbol_exprt &_univ_var,
+    const exprt &lower_bound,
+    const exprt &upper_bound,
+    const exprt &body);
 
   // Default bound inferior is 0
   string_constraintt(symbol_exprt univ_var, exprt upper_bound, exprt body)