Better use of std::enable_if (#1177)

* this commits adds tests that should fail, but don't

* Better use of std::enable_if

Replace the occurances of `class = std::enable_if_t<Cond>` and `typename = std::enable_if_t<Cond>` that have been identified in the previous commit with `std::enable_if_t<Cond, bool> = true`.

This commit is inspired by #1174, which changed one occurance in the owner header. This commit is aimed to fix all remaining occurances.

* fix failing checks

- core.cxx_gsl aktualisiert auf [](https://gitlab.avm.de/fos/repos/core.cxx_gsl/-/commit/)
- plc.access_lib aktualisiert auf [](https://gitlab.avm.de/fos/repos/plc.access_lib/-/commit/)
- plc.common aktualisiert auf [](https://gitlab.avm.de/fos/repos/plc.common/-/commit/)
- plc.daemon aktualisiert auf [](https://gitlab.avm.de/fos/repos/plc.daemon/-/commit/)
-

Test Plan:
-

---------

Co-authored-by: Werner Henze <werner.henze+gitcommits@posteo.de>
Co-authored-by: Werner Henze <w.henze@avm.de>

include/gsl/pointers

@@ -145,7 +145,7 @@ private:
     T ptr_;
 };
 
-template <typename T, typename = std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value>>
+template <typename T, std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value, bool> = true>
 void swap(not_null<T>& a, not_null<T>& b)
 {
     a.swap(b);