Record copy constructor should be protected

[cston]

The copy constructor is currently public which means it can be referenced unintentionally.

record A;
record B(object P) : A;

class Program
{
    static void Main()
    {
        _ = new B(null); // NullReferenceException calling B.B(B) rather than B.B(object)
    }
}

[jcouv]

Relates to #44902 (derived copy constructor should leverage base copy constructor)

[jcouv]

Fix merged into 16.7p3. Closing this