As suggested by @rnk here, I persisted the Map variable, previously in lld::coff::Writer::createSections(), now as a member in class lld::coff::Writer and converted its data into a PartialSection.
PartialSections are simply sections that contribute to an OutputSection.
This is needed later by D54802 to generate COFF groups in the * Linker * module.
I used a std::set below instead of a DenseSet because we need to retain a sorted order when enumerating.
It feels that UnmergedSection is still a bit weird name. You don't remove members from "Unmerged" once they are "merged" right?