Implemented optimization for BatchEmitOwnedPtrs that we only emit one complete

SerializedPtrID, followed by the *differences* in IDs. The big idea is that
most IDs will be just be 1 off from the previous (either that or NULL, which
we encode as a difference if 0), so this will greatly reduce the encoding
space for extra IDs to just 1 bit per pointer.

So far this optimization reduces serialization of Carbon.h by only 1%, but
we aren't using any abbreviations now in the Bitcode file to properly take
advantage of this optimization.

llvm-svn: 44199