Change all_objfiles_safe adapter to be a method on program_space

Change all_objfiles_safe adapter to be a method on program_space

This changes the all_objfiles_safe range adapter to be a method on the
program space, and fixes up all the users.

gdb/ChangeLog
2019-01-15  Tom Tromey  <tom@tromey.com>

	* progspace.h (program_space) <objfiles_safe_range>: New
	typedef.
	<objfiles_safe>: New method.
	* objfiles.h (class all_objfiles_safe): Remove.
	* objfiles.c (free_all_objfiles, objfile_purge_solibs): Update.
	* jit.c (jit_inferior_exit_hook): Update.