[CallPrinter] Adding heat coloring to CallPrinter
This patch introduces the heat coloring of the Call Printer which is based
on the relative "hotness" of each function. The patch is a part of sequence of
three patches, related to graphs Heat Coloring.
Another feature added is the flag similar to "-cfg-dot-filename-prefix",
which allows to write the graph into a named .pdf
Reviewers: rcorcs, apilipenko, davidxl, sfertile, fedor.sergeev, eraman, bollu
Differential Revision: https://reviews.llvm.org/D77172