Introduce the VTable interleaving scheme to the CFI design documentation

Dimitar et. al. in [1] proposed a novel VTable layout scheme that enables efficient implementation of virtual call CFI.

This patch adds an introduction of this scheme to the CFI design documentation.

[1] Protecting C++ Dynamic Dispatch Through VTable Interleaving. Dimitar Bounov, Rami Gökhan Kıcı, Sorin Lerner. https://cseweb.ucsd.edu/~lerner/papers/ivtbl-ndss16.pdf

Patch by Zhaomo Yang!

Differential Revision: https://reviews.llvm.org/D50372

llvm-svn: 341989