Extend CFGPrinter and CallPrinter with Heat Colors

Extends the CFGPrinter and CallPrinter with heat colors based on heuristics or
profiling information. The colors are enabled by default and can be toggled
on/off for CFGPrinter by using the option -cfg-heat-colors for both
-dot-cfg[-only] and -view-cfg[-only].  Similarly, the colors can be toggled
on/off for CallPrinter by using the option -callgraph-heat-colors for both
-dot-callgraph and -view-callgraph.

Patch by Rodrigo Caetano Rocha!

Differential Revision: https://reviews.llvm.org/D40425

llvm-svn: 335996