Fix multiple problems with RETURNING on a DML statement against a view,

Fix multiple problems with RETURNING on a DML statement against a view,
all inspired by [forum:/forumpost/dc3b92cfa0|forum post dc3b92cfa0].
(1) Do not allow a RETURNING clause to trick the code generator into thinking
that the view being updated has an INSTEAD OF trigger.
(2) Generate all result columns for a view in a DML statement.
(3) The automatic covering index for a view should cover all result columns
of the view.

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