* The `specifiers2` tests have a different result now due to the implementation
of CWG 2387.
* The `special_members/generated_copy` has improved results as the frontend has
improved support for deleted copy constructors.
Making `constexpr` imply `const` was correct for C++11 and was a correct
emulation of a GCC bug on GCC < 5.0. This test confirms that the problem
isn't there in C++20.