Warn on Activator.CreateInsatnce when ctor is not public.

[JohanLarsson]

class C
{
    internal C(int i) { }

    C M() => (C)Activator.CreateInstance(typeof(C), 1);
}

[JohanLarsson]

Not sure what the best code is if we write a fix, maybe:

class C
{
    internal C(int i) { }

    C M() => (C)Activator.CreateInstance(typeof(C), BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.CreateInstance, null, new object[] { 1 }, null, null);
}

[jnm2]

BindingFlags.CreateInstance is only relevant when calling the Type.InvokeMember method.
https://docs.microsoft.com/en-us/dotnet/api/system.reflection.bindingflags?view=netcore-3.1#System_Reflection_BindingFlags_CreateInstance