pgsql: Simplify planner's final setup of Aggrefs for partial aggregatio

Simplify planner's final setup of Aggrefs for partial aggregation.

Commit e06a38965's original coding for constructing the execution-time
expression tree for a combining aggregate was rather messy, involving
duplicating quite a lot of code in setrefs.c so that it could inject
a nonstandard matching rule for Aggrefs. Get rid of that in favor of
explicitly constructing a combining Aggref with a partial Aggref as input,
then allowing setref's normal matching logic to match the partial Aggref
to the output of the lower plan node and hence replace it with a Var.

In passing, rename and redocument make_partialgroup_input_target to have
some connection to what it actually does.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/59a3795c2589a0e6dfe4d9a886de9423b3f8b057

Modified Files
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src/backend/optimizer/plan/planner.c | 126 ++++++++++----
src/backend/optimizer/plan/setrefs.c | 325 +++++++++--------------------------
src/backend/optimizer/util/tlist.c | 51 ------
src/include/optimizer/planner.h | 2 +
src/include/optimizer/tlist.h | 1 -
5 files changed, 171 insertions(+), 334 deletions(-)