Fix async iterators to clear out state upon completion (#43522)

Fix async iterators to clear out state upon completion (#43522)

AsyncIteratorMethodBuilder was only doing its clean-up for completion (e.g. zeroing out the state machine and context, removing the object from a debugger-incited tracking table) if the last call to the iterator was part of asynchronous completion; if the last MoveNextAsync completed synchronously, the used code path could miss that cleanup work.