Change remote-sim.c to use type-safe registry

Change remote-sim.c to use type-safe registry

This changes remote-sim.c to use the type-safe registry.

2019-07-10  Tom Tromey  <tromey@adacore.com>

	* remote-sim.c (struct sim_inferior_data): Add initializers,
	constructor, and destructor.
	(sim_inferior_data_key): Change type.  Move lower.
	(check_for_duplicate_sim_descriptor): Update.
	(get_sim_inferior_data): Use new.  Update.
	(~sim_inferior_data_cleanup): Rename from
	sim_inferior_data_cleanup.  Simplify.
	(gdbsim_close_inferior, simulator_command)
	(sim_command_completer, _initialize_remote_sim): Update.
	(next_pid, INITIAL_PID): Move earlier.