pgsql: Test GROUP BY matching of join columns that are type-coerced by

Test GROUP BY matching of join columns that are type-coerced by USING.

If we have, say, an int column that is left-joined to a bigint column
with USING, the merged column is the int column promoted to bigint.
GROUP BY's tests for whether grouping on the merged column allows a
reference to the underlying column, or vice versa, should know about
that relationship --- and they do. But I nearly broke this case with
an ill-advised optimization, so the lack of any test coverage for it
seems like a bad idea.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/823e739d4a7257cf0ca58fc6eff3c4cec308fccf

Modified Files
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src/test/regress/expected/aggregates.out | 26 ++++++++++++++++++++++++++
src/test/regress/sql/aggregates.sql | 15 +++++++++++++++
2 files changed, 41 insertions(+)