[embedded] Emit weak_odr instead of linkonce_odr symbols under -Xfrontend -mergeable-symbols

[kubamracek]

We added a -Xfrontend -mergeable-symbols flag in #79242, see the description for the motivation behind that. We used LinkOnceODRLinkage for such mergeable symbols, but those are viewed as "removable if unused" by LLVM, which makes e.g. the entrypoint ("main") get removed from object files. Let's instead use WeakODRLinkage.

[kubamracek]

@swift-ci please test

[compnerd]

I think that this might be a problem for PE/COFF - there is no concept of a weak symbol. The way to handle this is to mark the symbol as link once odr and add a llvm.used for it to treat is as a GC root.

[rjmccall]

The distinction between weak_odr and linkonce_odr is purely a compile-time property; it shouldn't vary based on target. We do need to use a COMDAT on ELF and PE/COFF, though, if we want the linker to actually merge symbols.

[kubamracek]

ApplyIRLinkage already seems to apply a COMDAT for both weak_odr and linkonce_odr?

      if (IRL.Linkage == llvm::GlobalValue::LinkOnceODRLinkage ||
          IRL.Linkage == llvm::GlobalValue::WeakODRLinkage)
        if (Triple.supportsCOMDAT())
          if (llvm::GlobalObject *GO = dyn_cast<llvm::GlobalObject>(GV))
            GO->setComdat(M->getOrInsertComdat(GV->getName()));

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[kubamracek]

Given this is all under a hidden flag and meant for experimentations/explorations, I'm going to merge; will figure out about how to support PE/COFF if this behavior turns out to be something we want to actually use in practice for Embedded Swift.