We had an Assert() preventing whole-row expressions from being used in
the SET clause of INSERT ON CONFLICT, but it seems unnecessary, given
some tests, so remove it.  Add a new test to exercise the case.

Still at ExecInitPartitionInfo, we used map_partition_varattnos (which
constructs an attribute map, then calls map_variable_attnos) using
the same two relations many times in different expressions and with
different parameters.  Constructing the map over and over is a waste.
To avoid this repeated work, construct the map once, and use
map_variable_attnos() directly instead.

Author: Amit Langote, per comments by me (Álvaro)
Discussion: https://postgr.es/m/20180326142016.m4st5e34chrzrknk@alvherre.pgsql