This new instance type will be used for wrapper objects representing
exported exceptions. Currently the objects are empty and only serve as
an identity for exported exceptions. Eventually they will also need to
reference the signature underlying the exception to perform a signature
check upon import.

R=clemensh@chromium.org
TEST=mjsunit/wasm/exceptions-import
BUG=v8:8091

Change-Id: Ifdd561fc000090f4a985aeb45549fd7110849646
Reviewed-on: https://chromium-review.googlesource.com/1215166
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55752}