Make the objfile destructor private

Make the objfile destructor private

The idea behind this is that, in the long run, some code will need to
be able to hold onto an objfile after it is unlinked from the program
space.  In particular, this is needed for some functionality to be
moved to worker threads -- otherwise the objfile can be deleted while
still in use.

So, this makes ~objfile private, replacing it with an "unlink" method,
making it more obvious which operation is intended at the calling
points.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tom@tromey.com>

	* symfile.c (syms_from_objfile_1): Use objfile_up.
	(syms_from_objfile_1, remove_symbol_file_command): Call unlink
	method.
	(reread_symbols): Use objfile_up.
	* solib.c (update_solib_list, reload_shared_libraries_1): Call
	unlink method.
	* objfiles.h (struct objfile) <~objfile>: Now private.
	<unlink>: New method.
	(struct objfile_deleter): New.
	(objfile_up): New typedef.
	* objfiles.c (objfile::unlink): New method.
	(free_objfile_separate_debug, free_all_objfiles)
	(objfile_purge_solibs): Use it.
	* jit.c (jit_unregister_code): Remove.
	(jit_inferior_exit_hook, jit_event_handler): Call unlink on
	objfile.
	* compile/compile-object-run.c (do_module_cleanup): Call unlink on
	objfile.
	* compile/compile-object-load.c (compile_object_load): Use
	objfile_up.

Change-Id: I934bee70b26b8b24e1735828fb1e60fe8a05714f