This allows CoverageInfo to be distinguished from other kinds of
FixedArray at runtime. I also updated it to use untagged data since it
only stores ints, since that seems like the generally right thing to do
(even though I doubt anybody allocates enough of these to notice the
reduced GC work).

Related Torque changes:
- Allow structs containing untagged data to be used as class fields.
  This requires classifying them into the tagged or untagged sections of
  the class layout, and checking that their alignment requirements are
  met when stored in a packed array.
- Generate a struct containing struct field offsets, so we can ensure
  that the layouts defined in Torque and C++ code match. Of course it
  would be nice to generate a lot more (indexed accessors, synchronized
  accessors, GC visitors, etc.), but we can't do it all at once.

Change-Id: I29e2a2afe37e4805cd80e3a84ef9edfe7ca7bb6b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2047399Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#66318}