DebugInfo: Reuse common addresses for rnglist base address selections

This makes the offsets larger (since they are further from the base
address) but those are in the .dwo - and allows removing addresses and
relocations from the .o file.

This could be built into the AddressPool more fundamentally, perhaps -
when you ask for an AddressPool entry you could say "or give me some
other entry and an offset I need to use" - though what to do about
situations where the first use of an address in a section is not the
earliest address in that section... is tricky.

At least with range addresses we can be fairly sure we've seen the
earliest address first because we see the start address for the
function.

llvm-svn: 345224