RuntimeDyldMachO has the ability to keep track of

relocations (i.e., pieces of data whose addresses
are referred to elsewhere in the binary image) and
update the references when the section containing
the relocations moves.  The way this works is that
there is a map from section IDs to lists of
relocations.

Because the relocations are associated with the
section containing the data being referred to, they
are updated only when the target moves.  However,
many data references are relative and also depend
on the location of the referrer.

To solve this problem, I introduced a new data
structure, Referrer, which simply contains the
section being referred to and the index of the
relocation in that section.  These referrers are
associated with the source containing the
reference that needs to be updated, so now
regardless of which end of the relocation moves,
the relocation will now be updated correctly.

llvm-svn: 153147