Add warning wave for recursive generic struct

[YairHalberstadt]

There is an existing bug in CoreCLR (not mono or CoreRT) where the following legal code throws a type load exception:

static class Program
{
    static void Main()
    {
        var m = new M();
    }
}

struct N<T> { }
struct M { public N<M> E; }

The bug is unlikely to be fixed any time soon. See discussion starting at dotnet/runtime#6924 (comment).

I think this would be a good candidate for a warning wave so people don't accidentally write code, which whilst legal, won't actually run in the most popular runtime.

[333fred]

That seems like a fine thing to warn on, but should probably be raised as on issue on Roslyn instead.

[YairHalberstadt]

Can you move it please - I wasn't sure where warning waves go

[333fred]

I can't move a discussion to be an issue. :)

[YairHalberstadt]

Of course! Opened dotnet/roslyn#48741