Fix T81854: crash when undoing switch between sculpt and edit mode.

The logic of `BKE_sculpt_update_object_for_edit` was not correct. such
low-level functions should typically never preform depsgraph evaluation
themselves, they should be able to rely on getting a fully evaluated
depsgraph and just get needed data from there.

Supporting that required fixing other broken code higher in the
callstack, namely:
* `ED_object_sculptmode_enter_ex` was freeing evaluated data, for no
  valid reason it would seem.
* `sculpt_undosys_step_decode` was ensuring an evaluated depsgraph
  **before** calling `ED_object_mode_generic_exit`, which would
  invalidate a lot of evaluated data.

Note that it is fairly difficult to track down all code paths leading to
`BKE_sculpt_update_object_for_edit`, so there may be still cases where
this gets called with improperly evaluated depsgraph.

Reviewed By: sergey

Maniphest Tasks: T81854

Differential Revision: https://developer.blender.org/D9270

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