Improve async iterator exception stack traces (#21103)

Improve async iterator exception stack traces (#21103)

- The current stack trace prettying logic assumes that a method only has one StateMachineAttribute, but that's no longer true with async iterators, which (at least with the current implementation) can have both an AsyncStateMachineAttribute and an IteratorStateMachineAttribute.  As such, we change the prettying logic to look through all StateMachineAttributes rather than stopping at the first one.
- ManualResetValueTaskSourceCore.GetResult is responsible for propagating exceptions, but it's infrastructure and isn't particularly meaningful in a call stack.   Hide it from the stack trace, as we do for other such GetResult methods in Task and ValueTask.