[nextpnr_fpga_interchange] Proceed with place and route despite missing pin placement constraints

[clavin-xlnx]

Currently, the default behavior for nextpnr_fpga_interchange is to require pin location constraints in order to proceed with placement and routing. A nice feature would be to instead generate a set of pin location placements and proceed with a warning telling the user that they were missing and that the process is proceeding with manufactured locations.