Parse and dump PDB DBI Stream Header Information

The DBI stream contains a lot of bookkeeping information for other
streams. In particular it contains information about section contributions
and linked modules. This patch is a first attempt at parsing some of the
information out of the DBI stream. It currently only parses and dumps the
headers of the DBI stream, so none of the module data or section
contribution data is pulled out.

This is just a proof of concept that we understand the basic properties of
the DBI stream's metadata, and followup patches will try to extract more
detailed information out.

Differential Revision: http://reviews.llvm.org/D19500
Reviewed By: majnemer, ruiu

llvm-svn: 267585