Class declarations support structured heap data that is a subtype of
HeapObject. Only fields of Object subtypes (both strong and weak)
are currently supported (no scalar fields yet).

With this CL, both the field list macro used with the C++
DEFINE_FIELD_OFFSET_CONSTANTS macro (to make field offset constants) as
well as the Torque "operator '.field'" macros are generated for the
classes declared in Torque. This is a first step to removing the
substantial amount of duplication and boilerplate code
needed to declare heap object classes.

As a proof of concept, and handful of class field definitions,
including those for non trivial classes like JSFunction, have been
moved to Torque.

Bug: v8:7793
Change-Id: I2fa0b53db65fa6f5fe078fb94e1db3418f908753
Reviewed-on: https://chromium-review.googlesource.com/c/1373971
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58704}