pgsql: Add support for xmlval IS DOCUMENT expression.

Log Message:
-----------
Add support for xmlval IS DOCUMENT expression.

Modified Files:
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pgsql/src/backend/executor:
execQual.c (r1.206 -> r1.207)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c.diff?r1=1.206&r2=1.207)
pgsql/src/backend/parser:
gram.y (r2.573 -> r2.574)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.573&r2=2.574)
parse_expr.c (r1.207 -> r1.208)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c.diff?r1=1.207&r2=1.208)
parse_target.c (r1.152 -> r1.153)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_target.c.diff?r1=1.152&r2=1.153)
pgsql/src/backend/utils/adt:
ruleutils.c (r1.241 -> r1.242)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.241&r2=1.242)
xml.c (r1.16 -> r1.17)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.16&r2=1.17)
pgsql/src/include/nodes:
primnodes.h (r1.122 -> r1.123)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/primnodes.h.diff?r1=1.122&r2=1.123)
pgsql/src/include/utils:
xml.h (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h.diff?r1=1.9&r2=1.10)
pgsql/src/test/regress/expected:
xml.out (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml.out.diff?r1=1.6&r2=1.7)
xml_1.out (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml_1.out.diff?r1=1.7&r2=1.8)
pgsql/src/test/regress/sql:
xml.sql (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/xml.sql.diff?r1=1.6&r2=1.7)