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

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.