Parse PDB Name Hash Table

PDB has a lot of similar data structures.  We already have code
for parsing a Name Map, but PDB seems to have a different but
very similar structure that is a hash table.  This is the
beginning of code needed in order to parse the name hash table,
but it is not yet complete.  It parses the basic metadata of
the hash table, the bucket array, and the names buffer, but
doesn't use any of these fields yet as the data structure
requires a non-trivial amount of work to understand.

llvm-svn: 268268