Refactor raw pdb dumper into library

PDB parsing code was hand-rolled into llvm-pdbdump. This patch moves the
parsing of this code into DebugInfoPDB and makes the dumper use this.

This is achieved by implementing the skeleton of RawPdbSession, the
non-DIA counterpart to the existing PDB read interface. None of the type /
source file / etc information is accessible yet, so this implementation is
not yet close to achieving parity with the DIA counterpart, but the
RawSession class simply holds a reference to a PDBFile class which handles
parsing the file format. Additionally a PDBStream class is introduced
which allows accessing the bytes of a particular stream in a PDB file.

Differential Revision: http://reviews.llvm.org/D19343
Reviewed By: majnemer

llvm-svn: 267049