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Geometry Nodes: realize instances before passing geometry to constructive mesh modifier

Previously, when the output of a Geometry Nodes modifier would
contain instances, those could not be accessed by other existing
modifiers (e.g. the Array modifier). That is because those modifiers
don't know about instances.

Upcoming commits will improve an this in two ways:
* Also realize instances before deform modifiers.
* Convert a point cloud in the geometry to mesh vertices so that
they can be accessed as well.

Note, making instances real can result in loosing some information
that we do not support in Geometry Nodes yet. That includes some
special builtin attributes like bevel weights.

Ref T85281.

Differential Revision: https://developer.blender.org/D10432