[CFGPrinter] Adding heat coloring to CFGPrinter

This patch introduces the heat coloring of the Control Flow Graph which is based
on the relative "hotness" of each BB. The patch is a part of sequence of three
patches, related to graphs Heat Coloring.

Reviewers: rcorcs, apilipenko, davidxl, sfertile, fedor.sergeev, eraman, bollu

Differential Revision: https://reviews.llvm.org/D77161