As part of the work to implement catch prediction for async functions,
the resulting Promise that is the output of the function needs to be
available earlier for a couple reasons:
- To be able to do %DebugPushPromise/%DebugPopPromise over the body
  of the async function
- To be able to pass the resulting promise into AsyncFunctionAwait
  in order to set up the dependency chains

This patch creates the Promise earlier and pushes it onto the debug
stack; a later patch will set up the dependency chain. Although the
debug stack is set up, it's not anticipated that this will change
the catch prediction helpfully yet, as everything will still likely
be predicted as 'caught' for now, as before.

R=caitp@igalia.com,yangguo@chromium.org
CC=neis@chromium.org,gsathya@chromium.org
BUG=v8:5167

Review-Url: https://codereview.chromium.org/2233923003
Cr-Commit-Position: refs/heads/master@{#38957}