Reverted patch 44199:

http://llvm.org/viewvc/llvm-project?rev=44199&view=rev

This patch completely broke serialization due to an invariant I assumed but
did not hold. The assumed invariant was that all pointer IDs emitted by a call
to BatchEmitOwnedPtrs would be consecutive. This is only the case if there has
been no forward references to an owned pointer (and hence already registered
with the Serializer object).

llvm-svn: 44203