Access checks and copy elision before c++17

We claim to support guaranteed copy elision in C++03, but we still perform access checks for copy constructors (before c++17)
https://godbolt.org/z/WPEh8e94c

```cpp
class S {
public:
S(int, int);

// private:
S(const S &);
};

S s = S(0, 0);
```

If we make the constructor public, RVO is performed https://godbolt.org/z/WPEh8e94c

Aditionally `__cpp_guaranteed_copy_elision` is not set even if the documentation claim it should be
https://clang.llvm.org/docs/LanguageExtensions.html#id34

Maybe the documentation is just wrong.

Start with the reduced C++ example and the linked Godbolt results, comparing behavior before C++17 with and without an accessible copy constructor. Then inspect the Language Extensions documentation at the linked guaranteed-copy-elision section and the reported __cpp_guaranteed_copy_elision definition. Done means the access-check behavior and feature-test macro agree with the documented support, or the documentation is corrected.

Written by the indexing model from the issue text.