When looking up toJSON function presence on the BigInt prototype,
we must pass the original primitive value as the receiver to any
getter calls. Thanks to the magic of the LookupIterator, this is
actually easier than the alternative.

Bug: v8:9048
Change-Id: I72e144dbfa2862df479fd4deee40acd5dd468243
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1611538Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61475}