pgsql: Fix flattening of nested grouping sets.

Fix flattening of nested grouping sets.

Previously nested grouping set specifications accidentally weren't
flattened, but instead contained the nested specification as a element
in the outer list.

Fix this by, as actually documented in comments, concatenating the
nested set specification into the outer one. Also add tests to prevent
this from breaking again.

Author: Andrew Gierth, with tests from Jeevan Chalke
Reported-By: Jeevan Chalke
Discussion: CAM2+6=V5YvuxB+EyN4iH=GbD-XTA435TCNvnDFSD--YvXs+pww@mail.gmail.com
Backpatch: 9.5, where grouping sets were introduced

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/faab14ecb8c1b4ea2bee3723d4fa04f47275abd3

Modified Files
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src/backend/parser/parse_clause.c          |   13 ++-
src/test/regress/expected/groupingsets.out |  121 ++++++++++++++++++++++++++++
src/test/regress/sql/groupingsets.sql      |   29 +++++++
3 files changed, 160 insertions(+), 3 deletions(-)