[cfi-verify] Add DOT graph printing for GraphResult objects.

Allows users to view GraphResult objects in a DOT directed-graph format. This feature can be turned on through the --print-graphs flag.

Also enabled pretty-printing of instructions in output. Together these features make analysis of unprotected CF instructions much easier by providing a visual control flow graph.

Reviewers: pcc

Subscribers: llvm-commits, kcc, vlad.tsyrklevich

Differential Revision: https://reviews.llvm.org/D39819

llvm-svn: 318211