Remove IdentifierOrEnum and AwaitOrYield and reduce Callable to exclude enum.

Change-Id: Id80c0c5e7a8d4869ab8d841f3b973ea7c0c844cc
Reviewed-on: https://chromium-review.googlesource.com/c/1357044
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57992}