This change allows Torque code to initialize bitfield structs, using the
same syntax as struct initialization. It also moves the definition of
the JSPromise flags to Torque as an example usage.

Bug: v8:7793
Change-Id: I3d5e49aa22139ffb4b8ea9f308dd36a2d22b2c1b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2148176
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67338}