Recent Clang versions have enhanced -Wunused-but-set-variable which now
warns about these.

Bug: chromium:1309955
Change-Id: Id99e3eee60bf2c789e15251f65a192a6bf51f252
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3554603Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79645}