includes a patch from Matthias Kleine with a regression testcase!

Adds a new iterator 'data_iterator' to OnDiskHashTable which doesn't try to
reconstruct the external_key from the internal_key, which is useful for traits
that don't store enough information to do that mapping in their key. Also
deletes the 'item_iterator' from OnDiskHashTable as dead code.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154784 91177308-0d34-0410-b5e6-96231b3b80d8