Wasm compilation units got smaller and smaller with recent refactorings
(https://crrev.com/c/1587386, https://crrev.com/c/1587387,
https://crrev.com/c/1587388, plus previous CLs).
They now only store a function index and the requested compilation
tier. Hence there is no reason any more to heap-allocate them.
This CL changes the compilation unit queues and interfaces to store and
pass compilation units by value. Methods that could return an empty
{unique_ptr} before are now returning a {base::Optional}.

R=mstarzinger@chromium.org

Bug: v8:8343
Change-Id: I63037156b1a700095c13010450e5fedb51544401
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1588456
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61133}