[NFC][DWARFLinker] Refactor address emitting code.

As a preparation for implementing DWARFv5 address ranges generation,
this patch refactors existing address ranges generation code:
Split emitUnitRangesEntries into two functions emitDwarfDebugArangesTable
and emitDwarfDebugRangesTableFragment. Use AddressRanges to prepare linked
address ranges. Refactor Unit.getLowPc(), to use std::nullopt as undefined value.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D141943