pgsql: Fix build_grouping_chain() to not clobber its input lists.

Fix build_grouping_chain() to not clobber its input lists.

There's no good reason for stomping on the input data; it makes the logic
in this function no simpler, in fact probably the reverse. And it makes
it impossible to separate path generation from plan generation, as I'm
working towards doing; that will require more than one traversal of these
lists.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/a923af382c5678f3dfb591aacb6b90bf4e5ed7a9

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 116 ++++++++++++++++------------------
1 file changed, 54 insertions(+), 62 deletions(-)

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