translator: Fix C++11-deprecated register usage. Define 'register' as an empty string if we're in a modern version of C++. BUG=255186 BUG=angleproject:463 Change-Id: Ied044fb87a9b05c91cb419c54295c39f0f0ab776 Reviewed-on: https://chromium-review.googlesource.com/268512 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org>