BitcodeReader: Fix non-determinism in use-list order

`BasicBlockFwdRefs` (and `BlockAddrFwdRefs` before it) was being emptied
in a non-deterministic order.  When predicting use-list order I've
worked around this another way, but even when parsing lazily (and we
can't recreate use-list order) use-lists should be deterministic.

Make them so by using a side-queue of functions with forward-referenced
blocks that gets visited in order.

llvm-svn: 214899