We have to create WasmExportedFunction objects for any WebAssembly
function which may escape a WebAssembly instance. Up until now we
created these WasmExportedFunction objects eagerly during instantiation
time: for any exported function, and any element in an exported table we
create such an object.

With the anyref proposal, the table.get instruction can allow any
function in a table to escape its instance. Therefore we would have to
create a WasmExportedFunction object for any function which is put into
a table.

With this CL we create WasmExportedFunctions for table entries lazily.
We initialize tables with placeholders consisting of the instance and
the function index. If we encounter a placeholder in table.get, we
create the WasmExportedFunction for the expected function to return it.

R=mstarzinger@chromium.org
CC=titzer@chromium.org

Bug: v8:7581
Change-Id: I4f32bd7433285d0b04a22c0fb70b736bac55b3f1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1505575Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60115}