Add test for ByRefLike testing

[AaronRobinsonMSFT]

When constraints are across multiple generic parameters, they do not flow.

class C<T, U>
    where T: allows ref struct
    where U: T
{ }

interface I1 {}
ref struct S1 : I1 {}

new C<I1, S1>(); // Invalid.
new C<S1, S1>(); // Invalid.
new C<int, int>(); // Valid.

From the IL perspective, the fix would be to apply the ref struct constraint on both parameters.

class C<T, U>
    where T: allows ref struct
    where U: T, allows ref struct // Or whatever gesture is expected.
{ }

[AaronRobinsonMSFT]

/cc @jaredpar @AlekseyTs @lambdageek @fanyang-mono @davidwrighton

[lambdageek]

From the IL perspective, the fix would be to apply the ref struct constraint on both parameters.

I agree. I would expect that new C<I1, S1> is invalid because S1 is a ref struct and U does not allow ref structs - it doesn't matter whether T allows them or not.