This adds a constructor function to be used as the base class for all
exported functions. With type reflection enabled exported functions are
instances of this new constructor. Using the constructor directly as
well as the 'type' property is still missing.

R=jkummerow@chromium.org
TEST=mjsunit/wasm/type-reflection
BUG=v8:7742

Change-Id: Id58359cf7866d24be0745abf36a839d3d86e5922
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1617253
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61614}