pgsql: Preserve Var location information during flatten_join_alias_vars

Preserve Var location information during flatten_join_alias_vars.

This allows us to give correct syntax error pointers when complaining
about ungrouped variables in a join query with aggregates or GROUP BY.
It's pretty much irrelevant for the planner's use of the function, though
perhaps it might aid debugging sometimes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/391af9f7842ba8b8d2195aaf82879662434b97f3

Modified Files
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src/backend/optimizer/util/var.c |   17 ++++++++++-------
1 files changed, 10 insertions(+), 7 deletions(-)