Fix UPDATE/DELETE ... WHERE CURRENT OF on a table with virtual columns.

Fix UPDATE/DELETE ... WHERE CURRENT OF on a table with virtual columns.

Formerly, attempting to use WHERE CURRENT OF to update or delete from
a table with virtual generated columns would fail with the error
"WHERE CURRENT OF on a view is not implemented".

The reason was that the check preventing WHERE CURRENT OF from being
used on a view was in replace_rte_variables_mutator(), which presumed
that the only way it could get there was as part of rewriting a query
on a view. That is no longer the case, since replace_rte_variables()
is now also used to expand the virtual generated columns of a table.

Fix by doing the check for WHERE CURRENT OF on a view at parse time.
This is safe, since it is no longer possible for the relkind to change
after the query is parsed (as of b23cd185f).

Reported-by: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Author: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDc_TwzSgb=B_QgNLt3mvZdmRK23rLb+RkanSQkDF40GjA@mail.gmail.com
Backpatch-through: 18