Assigning to reference to std::unordered_set doesn't change the
reference. It merely replaces the content of rhs with that of lhs.
We should use pointers instead.

Bug: chromium:1056170
Change-Id: I496544ca4b16ce8ae8a9aff57cb05a07cad984c1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2412184Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69918}