This avoids the generation of fake external classes.

Bug: v8:7793
Change-Id: I9744b299d3ec474d72b298b4f6143f95e345d1d9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1625991Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61778}