The constructor_or_backpointer accessor of Map was not consistent with
the torque-defined field constructor_or_back_pointer_or_native_context,
leading to confusion. This CL brings them in sync, choosing the latter
spelling.

Change-Id: I3375c5f060bfd5e1e7cab195e3cca3d508c88154
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2674011
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72540}