This supports {valueOf() { return BigInt(...) }} objects as
operands in binary and unary operations.

Bug: v8:6791
Change-Id: I30ec9983ff611477742f9ea0bc6d04972fe21d3c
Reviewed-on: https://chromium-review.googlesource.com/716863
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48612}