Validate runtime async methods when unboxing stubs/instantiating stubs are involved

[jakobbotsch]

In particular we should validate that the stubs work properly when targeting async functions that take async continuation arguments.

[jakobbotsch]

@VSadov Can you take a look at this one? I think we need to change IL instantiating stubs/unboxing stubs to properly be runtime async methods when the target method is runtime async, and then we need to encode the calli signature with runtime async calling convention. It's possible a few JIT changes will be needed too.

[VSadov]

I will take a look.

[VSadov]

Since the stubs do not await, I think all that might be needed is to ensure that the calli in these stubs is always tail-called.