Teach users of OnDiskHashTable to define hash_value and offset types

This paves the way to making OnDiskHashTable work with hashes that are
not 32 bits wide and to making OnDiskHashTable work very large hash
tables. The LLVM change to use these types is upcoming.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206640 91177308-0d34-0410-b5e6-96231b3b80d8