Motivated by https://crrev.com/c/1196693, this allows to declare runtime
functions with return type never in Torque. For example:

extern runtime ReThrow(Context, Object) : never;

Change-Id: I5dd8fe0ca22c778364bfcf1caf52180039c5be7e
Reviewed-on: https://chromium-review.googlesource.com/c/1282957
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56709}