Implement the all_lookups_iterator for PCH as a follow-up to r153970. This

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.

llvm-svn: 154784