This adds experimental support for an 'except_ref' value type for caught
exceptions as per the exception handling proposal. In the current for it
is only allowed to have such types in the stack or in a local, support
for having it as part of any signature was left out.

The default value for a local of type 'except_ref' is the 'ref_null'
value for now. Since this value cannot escape a wasm function, the
concrete value is not actually observable.

R=ahaas@chromium.org
TEST=unittests/LocalDeclDecoderTest.ExceptRef,mjsunit/wasm/exceptions
BUG=v8:8091

Change-Id: I7bd65274327a833262f8749cbe0e24e737f6e0c1
Reviewed-on: https://chromium-review.googlesource.com/1196510Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55526}