Fixes parsing attributes of the form
```systemverilog
(* bottom_bound = 1'sh0 *)
sky130_fd_sc_hd__dfrtp_1 _1415_ (
.CLK(clk),
.D(in),
.Q(out),
.RESET_B(reset)
);
```
In particular "supporting" the signed indicator.
Co-authored-by: Mike Inouye <mikeinouye@google.com>
Signed-off-by: Ethan Mahintorabi <ethanmoon@google.com>