Vtable index type

[Mark Mitchell]

We should specify a type for vtable indices.  It seems to me that
`ptrdiff_t' is the obvious candidate.

This is important in that it limits the number of vtable entries we
can use.  (For example, g++ has historically used `short' for a vtable
index; that limited the size of vtables.)

--
Mark Mitchell                   mark at codesourcery.com
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