`foo ## __LINE__` just emits foo__LINE__ because of how preprocessor
expansion works.

The typical solution for this is to use a CONCAT macro, but we in fact
already have a helper for what this is trying to solve,
UNIQUE_IDENTIFIER, so just use that instead.

Change-Id: Icea3f01db458c5d557e0affd3b004f4478c6c315
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3293084
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77998}