OAv2 merge: prune unsupported or invalid subsections and attributes

During the merge, each subsection and attribute is assigned a status.
If the status is OK, the item is retained and serialized into the
output object. Otherwise, unsupported or invalid subsections and
attributes are pruned before serialization.

When a subsection or attribute is pruned, ld emits information-level
messages to inform the user, preventing confusion about the absence
of elements that were present in the input objects.