[dsymutil] Avoid invalid keep chains due to pruning

[JDevlieghere]

Summary

The pruning property that's part of the DIE info is fully computing during the analyzeContextInfo phase, but can be updated during the lookForDIEsToKeep phase.

// Keep a module forward declaration if there is no definition.
if (!(isODRAttribute(AttrSpec.Attr) && Info.Ctxt &&
      Info.Ctxt->hasCanonicalDIE()))
  Info.Prune = false;

When the pruning property is updated during the lookForDIEsToKeep phase, it's not propagated to the parent, unlike during the analyzeContextInfo phase. This can result in an invalid keep chain with the child DIE being marked as kept while its parent is still marked as pruned and therefore never kept, a situation that's now caught by 1b79bed8f532.

This patch fixes this issue by updating the pruning properties of the parent DIE during the parent walk.

Similar to c9cbe6937b1f, this issue was caught in a big internal project that doesn't lend itself to reduction and so far I haven't been able to create an artificial test case where (1) a pruned DIE doesn't have a decl context and (2) is marked as kept.

[avl]

llvm/lib/DWARFLinker/DWARFLinker.cpp

793

It looks OK. Though add the comment explaining what is going on, please?
That Prune is erased because we decided to keep module forward declaration(and its dependencies) inside lookForRefDIEsToKeep.

llvm/lib/DWARFLinker/DWARFLinker.cpp

case WorklistItemType::MarkODRCanonicalDie:
       continue;
     case WorklistItemType::LookForDIEsToKeep:
       break;
     }
 
     unsigned Idx = Current.CU.getOrigUnit().getDIEIndex(Current.Die);
     CompileUnit::DIEInfo &MyInfo = Current.CU.getInfo(Idx);
 
-    if (MyInfo.Prune)
+    if (MyInfo.Prune) {
+      // We're walking the dependencies of a module forward declaration that was
+      // kept because there is no definition.
+      if (Current.Flags & TF_DependencyWalk)
+        MyInfo.Prune = false;
+      else
         continue;
+    }
 
     // If the Keep flag is set, we are marking a required DIE's dependencies.
     // If our target is already marked as kept, we're all set.
     bool AlreadyKept = MyInfo.Keep;
     if ((Current.Flags & TF_DependencyWalk) && AlreadyKept)
       continue;
 
     // We must not call shouldKeepDIE while called from keepDIEAndDependencies,