Each Token was making a copy of the `val_str_`, despite the token being a slice on the original source.
Bug: tint:1383
Change-Id: I17b2da8f986ba105853aa47afe21bcc75f140f8e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/78320
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>