[analyzer] exploded-graph-rewriter: Initial commit.

This is a utility to improve readability and generally manipulate
GraphViz dumps of the analysis graph. Such dumps are often huge and
not only hard to read, but also often hang the viewer apps with their
mere size. Such script should significantly improve debugging experience.

Differential Revision: https://reviews.llvm.org/D62638

llvm-svn: 362340