[scanner] Explicitly add one-byte characters to a one-byte literal buffer when scanning ascii identifiers

Change-Id: Ia8337506d9f442d6887b6fa3c2ed17210ffd3c28
Reviewed-on: https://chromium-review.googlesource.com/1172789Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55096}