[clang] Ensure correct metadata for relative vtables

Prior to this, metadata pertaining to the size or address point offsets
into a relative vtable were twice the value they should be (treating
component widths as pointer width rather than 4 bytes). This prevented
some vtables from being devirtualized with D134320. This ensures the
correct metadata is written so whole program devirtualization can catch
these remaining devirt targets.

Differential Revision: https://reviews.llvm.org/D134687