Change how we manage groups in ICF.

Previously, on each iteration in ICF, we scan the entire vector of
input sections to find boundaries of groups having the same ID.

This patch changes the algorithm so that we now have a vector of ranges.
Each range contains a starting index and an ending index of the group.
So we no longer have to search boundaries on each iteration.

Performance-wise, this seems neutral. Instead of searching boundaries,
we now have to maintain ranges. But I think this is more readable
than the previous implementation.

Moreover, this makes easy to parallelize the main loop of ICF,
which I'll do in a follow-up patch.

llvm-svn: 288228