PCH: When deserializing an explicit "external definition", don't pass it to...

PCH: When deserializing an explicit "external definition", don't pass it to HandleTopLevelDecl -- this is already being done inside the reader.

This is something of a hack, since whether the reader actually did this depends on the "isConsumerInterestedIn" predicate. I think we need to rework how this works, but I need to discuss with Doug.

llvm-svn: 82111