[pdb] Add HashTable data structure.

[pdb] Add HashTable data structure.

This was being parsed / serialized ad-hoc inside the code
for a specific PDB stream.  But this data structure is used
in multiple ways / places within the PDB format.  To be able
to re-use it we need to raise this code out and make it more
generic.  In doing so, a number of bugs are fixed in the
original implementation, and support is added for growing
the hash table and deleting items from the hash table,
which had either been omitted or incorrect implemented in
the initial version.

Differential Revision: https://reviews.llvm.org/D28715

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292535 91177308-0d34-0410-b5e6-96231b3b80d8