pgsql: Define the right-hand input of AT TIME ZONE as a full a_expr

Log Message:
-----------
Define the right-hand input of AT TIME ZONE as a full a_expr instead of
c_expr.  Perhaps the restriction was once needed to avoid bison errors,
but it seems to work just fine now --- and even generates a slightly
smaller state machine.  This change allows examples like
SELECT '13:45'::timetz AT TIME ZONE '-07:00'::interval;
to work without parentheses around the right-hand input.

Modified Files:
--------------
    pgsql/src/backend/parser:
        gram.y (r2.487 -> r2.488)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.487&r2=2.488)