Implement some methods for NativeRawSymbol

This allows the ability to call IPDBSession::getGlobalScope with a NativeSession and
to then query it for some basic fields from the PDB's InfoStream.
Note that the symbols now have non-const references back to the Session so that
NativeRawSymbol can access the PDBFile through the Session.

Differential Revision: https://reviews.llvm.org/D30314

llvm-svn: 296049