This is the 1st step in series of CLs to move the SharedFunctionInfo
class to kNeverSerialized and make it concurrently accessible from
the background thread. This CL:
* Enables direct heap reads for the most basic members of SFI
  if FLAG_turbo_direct_heap_reads is enabled.
* Adds synchronization to SharedFunctionInfo::script_or_debug_info.

Bug: v8:7790
Change-Id: Ia7d28033e9053aae5771b1b9b174de40f194534d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2461238Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70877}