comments on type attributes broken in 9.1devel

The comment code refactoring c10575ff005c330d0475345621b7d381eb510c48
broke comments on composite type attributes:

COMMENT ON COLUMN foo.a is 'test';
ERROR:  42809: "foo" is a composite type
LOCATION:  heap_openrv, heapam.c:1110

This is because as the code was moved from CommentAttribute() to
get_object_address_attribute(), the call to relation_openrv() became a
heap_openrv(), which causes the above error.

Please check it out.