Runtime async should use async call conv for calli in async resumption stubs

[jakobbotsch]

#118301 introduces a new (internal) async calling convention that is a more robust way to represent that the calli has async calling convention. We should use it for the calli in async resumption stubs. Today that call gets marked as async based on the fact that there is a follow-up StubHelpers.AsyncCallContinuation() intrinsic, but that is not as robust.

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