This unifies and simplifies the way we instrument async functions for
the purpose of async stack traces and async stepping. It does so while
retaining the observable behavior on the inspector level (for now).

Previously we'd mark the implicit promise of the async function object
with the async task ID, and whenever we awaited, we'd copy the async
task ID to the throwaway promise that is created by the `await`. This
however made things unnecessarily interesting in the following regards:

1. We'd see `DebugDidHandle` and `DebugWillHandle` events after the
`AsyncFunctionFinished` events, coming from the throwaway promises,
while the implicit promise is "done". This is especially confusing
with rejection propagation and requires very complex stepping logic
for async functions (after this CL it'll be possible to unify and
simplify the stepping logic).
2. We have to thread through the "can suspend" information from the
Parser all the way through AsyncFunctionReject/AsyncFunctionResolve
to the async function instrumentation to decide whether to cancel the
pending task when the async function finishes.

This CL changes the instrumentation to only happen (non recurringly) for
the throwaway promises allocated upon `await`. This solves both problems
mentioned above, and works because upon the first `await` the stack
captured for the throwaway promise will include the synchronous part as
expected, while upon later `await`s the synchronous part will be empty
and the asynchronous part will be the stack captured for the previous
throwaway promise (and the V8Debugger automatically short circuits
stacks with empty synchronous part).

Bug: chromium:1280519, chromium:1277451, chromium:1246867
Change-Id: Id604dabc19ea133ea2e9dd63181b1fc33ccb5eda
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3383775Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78599}