add DG2Dot.h
add DG2Dot.h

Add class that prints dg to dot format. It's usefull when debugging
(non-LLVM) dep. graphs. Later it would be nice to use it in
llvm-dg-dump

Signed-off-by: Marek Chalupa <a05df156a69ad59faa9af9b666ef78009594c145@gmail.com>

mchalupa/dg,mchalupa/dg,mchalupa/dg,mchalupa/dg