EnumSet currently lives in src/utils.h, which is a conglomerate of many
different helper functions and classes. In order to remove unneeded
include, move the EnumSet to its own header.

R=titzer@chromium.org

Bug: v8:7490, v8:8562
Change-Id: I979814167e87b914e9807b03e342d8b34e514331
Reviewed-on: https://chromium-review.googlesource.com/c/1409430
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58818}