A class which is marked as internal or public can be visible outside
of the current file, where the use of the VWT indirectly is possible.
If the VWT is modified and inlined, it is possible that the offsets will
no longer match resulting in an invalid dispatch. Limit the pass to
when WMO is enabled or the type is private in non-WMO cases.
Private and internal classes shouldn't have ABI constraints on their concrete vtable layout, so if methods
don't have overrides in practice, we can elide their vtable entries.
We were not using the primary benefits of an intrusive list, namely the
ability to insert or remove from the middle of the list, so let's switch
to a plain vector. This also avoids linked-list pointer chasing.
Start with some high-level checks of whether a declaration is formally final, or has no visible overrides
in the domain of the program visible to the SIL module.
We can eventually adopt more of the logic from the Devirtualizer pass to tell whether call sites are
"effectively final", and maybe make the Devirtualizer consume that information applied by this pass.
