This adds support for having reference type values (i.e. anyref) stored
in an exception. It is the natural combination of the reference type
proposal and the exception handling proposal.

Note that this also introduces support for having write barriers in
generated WasmCode, as this is the first time we are storing references
within generated code. Such write barriers will be needed for other uses
of reference types (e.g. mutable global) regardless.

R=clemensh@chromium.org
TEST=mjsunit/wasm/exceptions-anyref
BUG=v8:8341

Change-Id: I1211d4a850954622cb873eede0b4024fecc3dd8b
Reviewed-on: https://chromium-review.googlesource.com/c/1296484
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56995}