Eliminate the old LateResolveValues data structure, replacing it with a

new, simpler, ForwardReferences data structure.  This is just the first
simple replacement, subsequent changes will improve the code more.

This simple change improves the performance of loading a file from HDF5
(contributed by Bill) from 2.36s to 1.93s, a 22% improvement.  This
presumably has to do with the fact that we only create ONE placeholder for
a particular forward referenced values, and also may be because the data
structure is much simpler.

llvm-svn: 8979