As one of several steps involved in supporting struct-valued fields
within classes, this CL generates type verification code for the data
contained in those structs.

In order to generate verification code, Torque needs to know about
struct field offsets and the total size of structs. Those calculations
are added to StructType itself and the function TypeVisitor::ComputeType
which initializes the StructType. I repurposed the Field::offset value
to behave in structs more like it does in classes (it had previously
indicated the index of a field within a struct, but nobody used that
value). Overall this works okay, and I think it's less confusing to have
Field::offset mean the same thing everywhere. However, some struct
fields have types with unknown size (Field::GetFieldSizeInformation
fails), so those fields are now marked with offset Field::kInvalidOffset
to indicate that the structs containing them should not be used within
class fields or anywhere else that requires packed layout.

Bug: v8:7793
Change-Id: If2677c8c81efc85e63b4bfb831d818a748427e18
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1897247
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65016}