Fix incorrect optimization of Activator.CreateInstance

[MichalStrehovsky]

We should not return "default" if there's a private parameterless constructor. GetDefaultConstructor only returns the default constructor in the C# sense (public parameterless ctor on a non-abstract class).

[VSadov]

Curious - what will happen if there is a parameterless constructor, but inaccessible. Will it throw an exception?

[MichalStrehovsky]

Yup, MissingMethodException.

[VSadov]

I think C# is still undecided what to do in this case. - Return default for compat reasons or make it a compile error.
It seems better if behaviors match. https://github.com/dotnet/csharplang/blob/main/proposals/parameterless-struct-constructors.md#object-creation

Behavior of Activator may tip the scales towards making this an error.
CC:@cston

[MichalStrehovsky]

AFAIK C# is tipping towards disallowing non-public parameterless constructors so this would avoid the problem.

I see it kind of as a runtime bug because the spec decided that any struct (regardless of its constructors) is going to satisfy the new constraint but we then went ahead and blocked it inconsistently for the only API where the constraint makes sense (Activator).