New utility to visualize optimization records

This is a new tool built on top of the new YAML ouput generated from
optimization remarks.  It produces HTML for easy navigation and
visualization.

The tool assumes that hotness information for the remarks is available
(the YAML file was produced with PGO).  It uses hotness to list the
remarks prioritized by the hotness on the index page.  Clicking the
source location of the remark in the list takes you the source where the
remarks are rendedered inline in the source.

For now, the tool is meant as prototype.

It's written in Python.  It uses PyYAML to parse the input.

Differential Revision: https://reviews.llvm.org/D25348

llvm-svn: 283571