[pdb] Add the ability to resolve TypeServer PDBs.

Some PDBs or object files can contain references to other PDBs
where the real type information lives.  When this happens,
all type indices in the original PDB are meaningless because
their records are not there.

With this patch we add the ability to pull type info from those
secondary PDBs.

Differential Revision: https://reviews.llvm.org/D29973

llvm-svn: 295382