Start the implementation of the shared heap write barrier by renaming
CLIENT_TO_SHARED to OLD_TO_SHARED. I planned to do this with the CL
introducing the write barrier but in order to keep that CL smaller do
this here already.

Bug: v8:11708
Change-Id: I204c728e333a4e80c30c0992e43c3cb6752fc660
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3468351Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79163}