This change implements support for reading and writing bitfields from
Torque code, and adds a couple of unit tests for this functionality. As
Tobias suggested, the LocationReference for a bitfield access contains
a nested LocationReference to where the bitfield struct is stored, so
that store operations can read the original value, update part of it,
and write it back.

Bug: v8:7793
Change-Id: I1004a5c7fcb6cf58df5ad50109b114bf89c80efc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1957841
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65487}