This switches the encoding of the exceptions (in the exceptions as well
as the import section) to use a signature index instead of a flat type
vector encoding. Note that only signatures that have a void return type
can be used for declaring exceptions.

R=clemensh@chromium.org
BUG=v8:8153

Change-Id: I481ccbce9ddf29becdf4ed7ceffe80d6145446e1
Reviewed-on: https://chromium-review.googlesource.com/c/1280323
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56654}