Thin link efficiency improvement. After adding an importing candidate to
the worklist we might have later added it again with a higher threshold.
Skip it when popped from the worklist if we recorded a higher threshold
than the current worklist entry, it will get processed again at the
higher threshold when that entry is popped.
This required adding the summary's GUID to the worklist, so that it can
be used to query the recorded highest threshold for it when we pop from the