MetadataLoader: replace the tracking of ForwardReferences and UnresolvedNodes...

MetadataLoader: replace the tracking of ForwardReferences and UnresolvedNodes with a set-based solution (NFC)

This makes it explicit what is the exact list to handle, and it
looks much more easy to manipulate and understand that the
previous custom tracking of min/max to express the range where
to look for.

Differential Revision: https://reviews.llvm.org/D28089

llvm-svn: 290507