Add a debug checker that is useful to understand how the ExplodedGraph is
built; it can be triggered using the following command:

 clang -cc1 -analyze -analyzer-checker=debug.ViewExplodedGraph my_program.c

A patch by Béatrice Creusillet!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184768 91177308-0d34-0410-b5e6-96231b3b80d8