.. and explicitly mark behavior through tags/naming conventions:

 // This method is never called when concurrent inlining is enabled.
 void SerializeFoo(NotConcurrentInliningTag);

 // This method is thread-safe and may be called at any time.
 void CacheBar();

It turns out that all our remaining SerializeFoo methods are already
either of the former or latter category and thus do not block removal
of the serialization phase for concurrent inlining.

Bug: v8:7790
Change-Id: If8f3bc2e407bc2824f83bfcd1f520f3b14dc58ec
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3026709
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75804}