Fix some minor issues in PDB parsing library.

1) Until now I'd never seen a valid PDB where the DBI stream and
   the PDB Stream disagreed on the "Age" field.  Because of that,
   we had code to assert that they matched.  Recently though I was
   given a PDB where they disagreed, so this assumption has proven
   to be incorrect.  Remove this check.

2) We were walking the entire list of hash values for types up front
   and then throwing away the values.  For large PDBs this was a
   significant slow down.  Remove this.

With this patch, I can dump the list of all compilands from a
1.5GB PDB file in just a few seconds.

llvm-svn: 303351