Wrap return value in std::move

This fixes #3238 (again).

Author: karkhaz

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);
 }