This introduces a dedicated instance type for exception tags. The main
motivation is to reduce their footprint and getting rid of a temporary
workaround that used the {JSObject} type for this purpose.

R=clemensh@chromium.org
BUG=v8:8091

Change-Id: Id5678bce513f2ac086c7380bd803011b11d5050e
Reviewed-on: https://chromium-review.googlesource.com/c/1354464Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57943}