Introduce the llvm-cfi-verify tool.

Summary: Introduces the llvm-cfi-verify tool to llvm. Includes the design document (docs/CFIVerify.rst). Current implementation of the tool is simply a disassembler that identifies and prints the indirect control flow instructions.

Reviewers: vlad.tsyrklevich

Reviewed By: vlad.tsyrklevich

Patch by Mitch Phillips

Subscribers: llvm-commits, kcc, pcc, mgorny

Differential Revision: https://reviews.llvm.org/D37937

llvm-svn: 313688