[pdb] Correctly parse the hash adjusters table from TPI stream.

This is not a list of pairs, it is a hash table data structure. We now
correctly parse this out and dump it from llvm-pdbdump.

We still need to understand the conditions that lead to a type
getting an entry in the hash adjuster table.  That will be done
in a followup investigation / patch.

Differential Revision: https://reviews.llvm.org/D29090

llvm-svn: 293090