pgsql: Move SYMMETRIC/ASYMMETRIC to reserved words to avoid shift/reduce

Log Message:
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Move SYMMETRIC/ASYMMETRIC to reserved words to avoid shift/reduce conflicts.

Modified Files:
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pgsql/src/backend/parser:
gram.y (r2.493 -> r2.494)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.493&r2=2.494)