Wrap return value in std::move

This fixes #3238 (again).

Author: karkhaz

jbmc/src/java_bytecode/java_string_library_preprocess.cpp

@@ -1251,14 +1251,14 @@ exprt java_string_library_preprocesst::get_primitive_value_of_object(
         typecast_exprt(object, pointer_type), pointer_type.subtype());
       member_exprt deref_value(deref, value_comp.get_name(), value_comp.type());
       code.add(code_assignt(value, deref_value), loc);
-      return value;
+      return std::move(value);
     }
   }
 
   warning() << object_type->get_identifier()
             << " not available to format function" << eom;
   code.add(code_declt(value), loc);
-  return value;
+  return std::move(value);
 }
 
 /// Helper for format function. Returns the expression:

src/ansi-c/literals/convert_float_literal.cpp

@@ -101,5 +101,5 @@ exprt convert_float_literal(const std::string &src)
     return complex_exprt(zero_real_component, result, complex_type);
   }
 
-  return result;
+  return std::move(result);
 }