[pdb] added support for dumping globals stream

Summary: This adds support for dumping the globals stream from PDB files using llvm-pdbdump, similar to the support we have for the publics stream.

Reviewers: ruiu, zturner

Subscribers: beanz, mgorny, modocache

Differential Revision: https://reviews.llvm.org/D25801

llvm-svn: 284861