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
------
REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b17ae36ba9521014c5ae30cb3a3f77c439b41bb3

Modified Files
--------------
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(-)