pdbdump: Print "Publics" stream.

Publics stream seems to contain information as to public symbols.
It actually contains a serialized hash table along with fixed-sized
headers. This patch is not complete. It scans only till the end of
the stream and dump the header information. I'll write code to
de-serialize the hash table later.

Reviewers: zturner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20256

llvm-svn: 269484