[dsymutil] Avoid invalid keep chains due to pruning

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 1b79bed8.

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

Differential revision: https://reviews.llvm.org/D140930