Introduce the llvm-cfi-verify tool (resubmission of D37937).

Summary: Resubmission of D37937. Fixed i386 target building (conversion from std::size_t& to uint64_t& failed). Fixed documentation warning failure about docs/CFIVerify.rst not being in the tree.

Reviewers: vlad.tsyrklevich

Reviewed By: vlad.tsyrklevich

Patch by Mitch Phillips

Subscribers: sbc100, mgorny, pcc, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D38089

llvm-svn: 313809