Fix ON CONFLICT bugs that manifest when used in rules.

Andres Freund <andres@anarazel.de>

Commit: 4af6e61a363246cf7fff3368a76603b0ce9945dd
Author: Andres Freund <andres@anarazel.de>
Date: 2015-05-12T22:13:22Z
Releases: 9.5.0
Fix ON CONFLICT bugs that manifest when used in rules.

Specifically the tlist and rti of the pseudo "excluded" relation weren't
properly treated by expression_tree_walker, which lead to errors when
excluded was referenced inside a rule because the varnos where not
properly adjusted.  Similar omissions in OffsetVarNodes and
expression_tree_mutator had less impact, but should obviously be fixed
nonetheless.

A couple tests of for ON CONFLICT UPDATE into INSERT rule bearing
relations have been added.

In passing I updated a couple comments.

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