MSVC doesn't define __cplusplus appropriately, so use _MSVC_LANG instead.

Bug: chromium:1274247
Change-Id: I40ef9f42f4e55b3410f89ff23c228f1b1201c4ef
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3308423
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Adam Klein <adamk@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78180}